diff --git a/m4/gccoptim.m4 b/m4/gccoptim.m4
index 07753658b..beab5ba84 100644
--- a/m4/gccoptim.m4
+++ b/m4/gccoptim.m4
@@ -49,7 +49,7 @@ EOF
   done
   rm -f conftest*
   AC_LANG_POP([C++])
-  CXXFLAGS="$cf_save_CFLAGS $ac_cv_prog_gxx_opt_flags"])
+  CXXFLAGS="$cf_save_CXXFLAGS $ac_cv_prog_gxx_opt_flags"])
 else
   case $enable_optimizations in
     no) ;;