diff --git a/configure.ac b/configure.ac
index 6dc7c698..eebe10a1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -46,7 +46,7 @@ dnl note that we don't just check if the compiler accepts '-std=x11'
 dnl but also that it supports the _Thread_local keyword because some compilers
 dnl (e.g. gcc 4.8) accept the command line option but do not implement TLS
 saved_CFLAGS="${CFLAGS}"
-CFLAGS="-std=gnu11"
+CFLAGS="${saved_CFLAGS} -std=gnu11"
 AC_MSG_CHECKING([if $CC supports -std=gnu11])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([_Thread_local int x;], [x = 42;])],
 	[AC_MSG_RESULT([yes])
@@ -55,7 +55,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([_Thread_local int x;], [x = 42;])],
 	 c_dialect=])
 if test "x$c_dialect" != xgnu; then
 	dnl fallback check for -std=c11 compiler support (required)
-	CFLAGS="-std=c11"
+	CFLAGS="${saved_CFLAGS} -std=c11"
 	AC_MSG_CHECKING([if $CC supports -std=c11])
 	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([_Thread_local int x;], [x = 42;])],
 		[AC_MSG_RESULT([yes])],