--- a/build/autoconf/toolchain.m4
+++ b/build/autoconf/toolchain.m4
@@ -68,16 +68,24 @@ CLANG_CL=
 if test "$compiler" = "clang"; then
     GNU_CC=1
     GNU_CXX=1
     CLANG_CC=1
     CLANG_CXX=1
 fi
 if test "$compiler" = "clang-cl"; then
     CLANG_CL=1
+    # We force clang-cl to emulate Visual C++ 2013 in configure.in, but that
+    # is based on the CLANG_CL variable defined here, so make sure that we're
+    # getting the right version here manually.
+    CC_VERSION=1800
+    CXX_VERSION=1800
+    # Build on clang-cl with MSVC 2013 with fallback emulation.
+    CFLAGS="$CFLAGS -fmsc-version=1800 -fallback"
+    CXXFLAGS="$CXXFLAGS -fmsc-version=1800 -fallback"
 fi
 
 if test "$GNU_CC"; then
     if `$CC -print-prog-name=ld` -v 2>&1 | grep -c GNU >/dev/null; then
         GCC_USE_GNU_LD=1
     fi
 fi
 

--- a/configure.in
+++ b/configure.in
@@ -509,20 +509,17 @@ case "$target" in
         AC_SUBST(MSVS_VERSION)
         AC_SUBST(MSVC_C_RUNTIME_DLL)
         AC_SUBST(MSVC_CXX_RUNTIME_DLL)
 
         # Disable SEH on clang-cl because it doesn't implement them yet.
         if test -z "$CLANG_CL"; then
             AC_DEFINE(HAVE_SEH_EXCEPTIONS)
         else
-            # Build on clang-cl with MSVC 2013 with fallback emulation.
-            CFLAGS="$CFLAGS -fmsc-version=1800 -fallback"
-            CXXFLAGS="$CXXFLAGS -fmsc-version=1800 -fallback"
-            # Send our CFLAGS to NSS too
+            # Send our CFLAGS to NSS
             MOZ_CFLAGS_NSS=1
             AC_DEFINE_UNQUOTED(GTEST_HAS_SEH, 0)
         fi
 
         if test -n "$WIN32_REDIST_DIR"; then
           if test ! -d "$WIN32_REDIST_DIR"; then
             AC_MSG_ERROR([Invalid Win32 Redist directory: ${WIN32_REDIST_DIR}])
           fi