--- a/configure.in
+++ b/configure.in
@@ -1407,25 +1407,27 @@ if test "$GNU_CC"; then
                 else
                     DSO_LDOPTS="$DSO_LDOPTS -Wl,--warn-unresolved-symbols"
                 fi
                 ;;
             esac
         fi
     fi
 
-    # Turn on GNU-specific warnings:
+    # Turn on gcc/clang warnings:
+    # https://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Warning-Options.html
+    #
     # -Wall - turn on a lot of warnings
-    # -Wpointer-arith - good to have
     # -Wdeclaration-after-statement - MSVC doesn't like these
-    # -Werror=return-type - catches missing returns, zero false positives
-    # -Werror=int-to-pointer-cast - catches cast to pointer from integer of different size
+    # -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
-    # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
-    # -Wsign-compare - catches comparison of signed and unsigned types
     #
     _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)
 
@@ -1473,25 +1475,27 @@ else
     DSO_PIC_CFLAGS='-KPIC'
     _DEFINES_CFLAGS='$(ACDEFINES) -D_MOZILLA_CONFIG_H_ -DMOZILLA_CLIENT'
 fi
 
 if test "$GNU_CXX"; then
     # FIXME: Let us build with strict aliasing. bug 414641.
     CXXFLAGS="$CXXFLAGS -fno-exceptions -fno-strict-aliasing"
 
-    # Turn on GNU-specific warnings:
+    # Turn on gcc/clang warnings:
+    # https://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Warning-Options.html
+    #
     # -Wall - turn on a lot of warnings
-    # -Wpointer-arith - good to have
-    # -Woverloaded-virtual - ???
-    # -Werror=return-type - catches missing returns, zero false positives
-    # -Werror=int-to-pointer-cast - catches cast to pointer from integer of different size
-    # -Werror=type-limits - catches overflow bugs, few false positives
     # -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)
 

--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -1159,25 +1159,27 @@ if test "$GNU_CC"; then
     AC_MSG_CHECKING([for --build-id option to ld])
     _SAVE_LDFLAGS=$LDFLAGS
     LDFLAGS="$LDFLAGS -Wl,--build-id"
     AC_TRY_LINK(,,AC_MSG_RESULT([yes])
                   [NSPR_LDFLAGS="$NSPR_LDFLAGS -Wl,--build-id"],
                   AC_MSG_RESULT([no])
                   LDFLAGS=$_SAVE_LDFLAGS)
 
-    # Turn on GNU-specific warnings:
+    # Turn on gcc/clang warnings:
+    # https://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Warning-Options.html
+    #
     # -Wall - turn on a lot of warnings
-    # -Wpointer-arith - good to have
     # -Wdeclaration-after-statement - MSVC doesn't like these
-    # -Werror=return-type - catches missing returns, zero false positives
-    # -Werror=int-to-pointer-cast - catches cast to pointer from integer of different size
+    # -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
-    # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
-    # -Wsign-compare - catches comparison of signed and unsigned types
     #
     _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)
 
@@ -1221,26 +1223,28 @@ else
     fi
 
     DSO_CFLAGS=''
     DSO_PIC_CFLAGS='-KPIC'
     _DEFINES_CFLAGS='$(ACDEFINES) -D_JS_CONFDEFS_H_ -DMOZILLA_CLIENT'
 fi
 
 if test "$GNU_CXX"; then
-    # Turn on GNU-specific warnings:
+    # Turn on gcc/clang warnings:
+    # https://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Warning-Options.html
+    #
     # -Wall - turn on a lot of warnings
-    # -Wpointer-arith - good to have
-    # -Woverloaded-virtual - ???
-    # -Werror=return-type - catches missing returns, zero false positives
-    # -Werror=int-to-pointer-cast - catches cast to pointer from integer of different size
+    # -Wconversion-null - catches conversions between NULL and non-pointer types
+    # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
+    # -Wint-to-pointer-cast - catches cast to pointer from integer of different size
+    # -Woverloaded-virtual - function declaration hides virtual function from base class
+    # -Wpointer-arith - catches pointer arithmetic using NULL or sizeof(void)
+    # -Wreturn-type - catches missing returns, zero false positives
+    # -Wsign-compare - catches comparison of signed and unsigned types
     # -Wtype-limits - catches overflow bugs, few false positives
-    # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
-    # -Werror=conversion-null - catches conversions between NULL and non-pointer types
-    # -Wsign-compare - catches comparison of signed and unsigned types
     #
     _WARNINGS_CXXFLAGS="${_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)
     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)