diff --git a/config/ac_cxx_warn_effective_cxx.m4 b/config/ac_cxx_warn_effective_cxx.m4
index 317ad1d..8085d24 100644
--- a/config/ac_cxx_warn_effective_cxx.m4
+++ b/config/ac_cxx_warn_effective_cxx.m4
@@ -10,19 +10,19 @@ AC_DEFUN([HAVE_WARN_EFFECTIVE_CXX],
 [
 AC_REQUIRE([AC_PROG_CXX])
 AC_MSG_CHECKING([whether the C++ compiler (${CXX}) accepts -Weffc++])
-AC_CACHE_VAL([cv_warn_effective_cxx],
+AC_CACHE_VAL([_cv_warn_effective_cxx],
 [
 AC_LANG_SAVE
 AC_LANG_CPLUSPLUS
 save_cxxflags="$CXXFLAGS"
 CXXFLAGS="$CXXFLAGS -Weffc++"
 AC_TRY_COMPILE([],[main();],
-[cv_warn_effective_cxx=yes], [cv_warn_effective_cxx=no])
+[_cv_warn_effective_cxx=yes], [_cv_warn_effective_cxx=no])
 CXXFLAGS="$save_cxxflags"
 AC_LANG_RESTORE
 ])
-AC_MSG_RESULT([$cv_warn_effective_cxx])
-if test "$cv_warn_effective_cxx" = yes; then
+AC_MSG_RESULT([$_cv_warn_effective_cxx])
+if test "$_cv_warn_effective_cxx" = yes; then
 	WARN_EFFECTIVE_CXX=-Weffc++
 	WARN_NO_EFFECTIVE_CXX=-Wno-effc++
 fi