--- a/build/autoconf/libstdcxx.py
+++ b/build/autoconf/libstdcxx.py
@@ -62,12 +62,12 @@ def find_version(e):
 
     p = subprocess.Popen(['readelf', '-V', libstdcxx], stdout=subprocess.PIPE)
     versions = [parse_readelf_line(x)
                 for x in p.stdout.readlines() if 'Name: GLIBCXX' in x]
     last_version = sorted(versions, cmp = cmp_ver)[-1]
     return encode_ver(last_version)
 
 if __name__ == '__main__':
-    cxx_env = os.environ.get('CXX', 'c++')
+    cxx_env = os.environ['CXX']
     print 'MOZ_LIBSTDCXX_TARGET_VERSION=%s' % find_version(cxx_env)
     host_cxx_env = os.environ.get('HOST_CXX', cxx_env)
     print 'MOZ_LIBSTDCXX_HOST_VERSION=%s' % find_version(host_cxx_env)

--- a/configure.in
+++ b/configure.in
@@ -7693,17 +7693,17 @@ dnl ====================================
 STDCXX_COMPAT=
 MOZ_ARG_ENABLE_BOOL(stdcxx-compat,
 [  --enable-stdcxx-compat  Enable compatibility with older libstdc++],
     STDCXX_COMPAT=stdc++compat.cpp)
 
 AC_SUBST(STDCXX_COMPAT)
 
 if test -n "$STDCXX_COMPAT"; then
-   eval $($PYTHON $_topsrcdir/build/autoconf/libstdcxx.py)
+   eval $(CXX="$CXX" $PYTHON $_topsrcdir/build/autoconf/libstdcxx.py)
    AC_SUBST(MOZ_LIBSTDCXX_TARGET_VERSION)
    AC_SUBST(MOZ_LIBSTDCXX_HOST_VERSION)
 fi
 
 dnl ========================================================
 dnl = 
 dnl = Profiling and Instrumenting
 dnl =