diff --git a/build/autoconf/libstdcxx.py b/build/autoconf/libstdcxx.py
index cc4ecb678938..35008918e6d9 100755
--- a/build/autoconf/libstdcxx.py
+++ b/build/autoconf/libstdcxx.py
@@ -67,7 +67,7 @@ def find_version(e):
     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)
diff --git a/configure.in b/configure.in
index 7a1d9d5a49c5..331ae0dc8037 100644
--- a/configure.in
+++ b/configure.in
@@ -7698,7 +7698,7 @@ MOZ_ARG_ENABLE_BOOL(stdcxx-compat,
 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