--- a/configure.in
+++ b/configure.in
@@ -1419,22 +1419,26 @@ if test "$GNU_CC"; then
     # -Wdeclaration-after-statement - MSVC doesn't like these
     # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
     # -Wint-to-pointer-cast - catches cast to pointer from integer of different size
     # -Wpointer-arith - catches pointer arithmetic using NULL or sizeof(void)
     # -Wreturn-type - catches missing returns, zero false positives
     # -Wsign-compare - catches comparison of signed and unsigned types
     # -Wtype-limits - catches overflow bugs, few false positives
     #
-    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wall -Wpointer-arith -Wdeclaration-after-statement"
-    MOZ_C_SUPPORTS_WARNING(-W, error=return-type, ac_c_has_werror_return_type)
-    MOZ_C_SUPPORTS_WARNING(-W, error=int-to-pointer-cast, ac_c_has_werror_int_to_pointer_cast)
-    MOZ_C_SUPPORTS_WARNING(-W, type-limits, ac_c_has_wtype_limits)
-    MOZ_C_SUPPORTS_WARNING(-W, empty-body, ac_c_has_wempty_body)
-    MOZ_C_SUPPORTS_WARNING(-W, sign-compare, ac_c_has_sign_compare)
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wall"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wdeclaration-after-statement"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wempty-body"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wpointer-arith"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wsign-compare"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wtype-limits"
+
+    # Treat some warnings as errors:
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Werror=int-to-pointer-cast"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Werror=return-type"
 
     # Turn off the following warnings that -Wall turns on:
     # -Wno-unused - lots of violations in third-party code
     #
     _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wno-unused"
 
     if test -z "$INTEL_CC" -a -z "$CLANG_CC"; then
        # Don't use -Wcast-align with ICC or clang
@@ -1487,30 +1491,35 @@ if test "$GNU_CXX"; then
     # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
     # -Wint-to-pointer-cast - catches cast to pointer from integer of different size
     # -Woverloaded-virtual - function declaration hides virtual function from base class
     # -Wpointer-arith - catches pointer arithmetic using NULL or sizeof(void)
     # -Wreturn-type - catches missing returns, zero false positives
     # -Wsign-compare - catches comparison of signed and unsigned types
     # -Wtype-limits - catches overflow bugs, few false positives
     #
-    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wall -Wpointer-arith -Woverloaded-virtual"
-    MOZ_CXX_SUPPORTS_WARNING(-W, error=return-type, ac_cxx_has_werror_return_type)
-    MOZ_CXX_SUPPORTS_WARNING(-W, error=int-to-pointer-cast, ac_cxx_has_werror_int_to_pointer_cast)
-    MOZ_CXX_SUPPORTS_WARNING(-W, error=type-limits, ac_cxx_has_werror_type_limits)
-    MOZ_CXX_SUPPORTS_WARNING(-W, empty-body, ac_cxx_has_wempty_body)
-    MOZ_CXX_SUPPORTS_WARNING(-W, sign-compare, ac_cxx_has_sign_compare)
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wall"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wempty-body"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Woverloaded-virtual"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wpointer-arith"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wsign-compare"
+
+    # Treat some warnings as errors:
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Werror=int-to-pointer-cast"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Werror=return-type"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Werror=type-limits"
 
     # Turn off the following warnings that -Wall turns on:
     # -Wno-invalid-offsetof - we use offsetof on non-POD types frequently
     # -Wno-inline-new-delete - we inline 'new' and 'delete' in mozalloc
     #   for performance reasons, and because GCC and clang accept it (though
     #   clang warns about it).
     #
-    MOZ_CXX_SUPPORTS_WARNING(-Wno-, invalid-offsetof, ac_cxx_has_wno_invalid_offsetof)
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wno-invalid-offsetof"
+
     MOZ_CXX_SUPPORTS_WARNING(-Wno-, inline-new-delete, ac_cxx_has_wno_inline_new_delete)
 
     if test -z "$INTEL_CXX" -a -z "$CLANG_CXX"; then
        # Don't use -Wcast-align with ICC or clang
        case "$CPU_ARCH" in
            # And don't use it on hppa, ia64, sparc, arm, since it's noisy there
            hppa | ia64 | sparc | arm)
            ;;

--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -1171,22 +1171,26 @@ if test "$GNU_CC"; then
     # -Wdeclaration-after-statement - MSVC doesn't like these
     # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
     # -Wint-to-pointer-cast - catches cast to pointer from integer of different size
     # -Wpointer-arith - catches pointer arithmetic using NULL or sizeof(void)
     # -Wreturn-type - catches missing returns, zero false positives
     # -Wsign-compare - catches comparison of signed and unsigned types
     # -Wtype-limits - catches overflow bugs, few false positives
     #
-    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wall -Wpointer-arith -Wdeclaration-after-statement"
-    MOZ_C_SUPPORTS_WARNING(-W, error=return-type, ac_c_has_werror_return_type)
-    MOZ_C_SUPPORTS_WARNING(-W, error=int-to-pointer-cast, ac_c_has_werror_int_to_pointer_cast)
-    MOZ_C_SUPPORTS_WARNING(-W, type-limits, ac_c_has_wtype_limits)
-    MOZ_C_SUPPORTS_WARNING(-W, empty-body, ac_c_has_wempty_body)
-    MOZ_C_SUPPORTS_WARNING(-W, sign-compare, ac_c_has_sign_compare)
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wall"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wdeclaration-after-statement"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wempty-body"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wpointer-arith"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wsign-compare"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wtype-limits"
+
+    # Treat some warnings as errors:
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Werror=int-to-pointer-cast"
+    _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Werror=return-type"
 
     # Turn off the following warnings that -Wall turns on:
     # -Wno-unused - lots of violations in third-party code
     #
     _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} -Wno-unused"
 
     if test -z "$INTEL_CC" -a -z "$CLANG_CC"; then
        # Don't use -Wcast-align with ICC or clang
@@ -1236,28 +1240,33 @@ if test "$GNU_CXX"; then
     # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
     # -Wint-to-pointer-cast - catches cast to pointer from integer of different size
     # -Woverloaded-virtual - function declaration hides virtual function from base class
     # -Wpointer-arith - catches pointer arithmetic using NULL or sizeof(void)
     # -Wreturn-type - catches missing returns, zero false positives
     # -Wsign-compare - catches comparison of signed and unsigned types
     # -Wtype-limits - catches overflow bugs, few false positives
     #
-    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wall -Wpointer-arith -Woverloaded-virtual"
-    MOZ_CXX_SUPPORTS_WARNING(-W, error=return-type, ac_cxx_has_werror_return_type)
-    MOZ_CXX_SUPPORTS_WARNING(-W, error=int-to-pointer-cast, ac_cxx_has_werror_int_to_pointer_cast)
-    MOZ_CXX_SUPPORTS_WARNING(-W, type-limits, ac_cxx_has_wtype_limits)
-    MOZ_CXX_SUPPORTS_WARNING(-W, empty-body, ac_cxx_has_wempty_body)
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wall"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wempty-body"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Woverloaded-virtual"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wpointer-arith"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wsign-compare"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wtype-limits"
+
+    # Treat some warnings as errors:
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Werror=int-to-pointer-cast"
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Werror=return-type"
+
     MOZ_CXX_SUPPORTS_WARNING(-W, error=conversion-null, ac_cxx_has_werror_conversion_null)
-    MOZ_CXX_SUPPORTS_WARNING(-W, sign-compare, ac_cxx_has_sign_compare)
 
     # Turn off the following warnings that -Wall turns on:
     # -Wno-invalid-offsetof - we use offsetof on non-POD types frequently
     #
-    MOZ_CXX_SUPPORTS_WARNING(-Wno-, invalid-offsetof, ac_cxx_has_wno_invalid_offsetof)
+    _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wno-invalid-offsetof"
 
     if test -z "$INTEL_CXX" -a -z "$CLANG_CXX"; then
        # Don't use -Wcast-align with ICC or clang
        case "$CPU_ARCH" in
            # And don't use it on hppa, ia64, sparc, arm, since it's noisy there
            hppa | ia64 | sparc | arm)
            ;;
            *)