diff --git a/configure.ac b/configure.ac
index 5406b4e..8cbd8c7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -32,7 +32,7 @@ AC_ARG_WITH([libdivsufsort],
 
 AS_IF([test "x$with_libdivsufsort" != "xno"],
 	[
-		CFLAGS="-fms-extensions"
+		CFLAGS="$CFLAGS -fms-extensions"
 		AC_CHECK_HEADERS([divsufsort.h],[have_libdivsufsort=yes],[have_libdivsufsort=no])
 		AC_CHECK_LIB(divsufsort, divsufsort, [], [have_libdivsufsort=no])
 	],