configure.ac: set -std=gnu11

Change-Id: I4bf2f1cb724bc3e3c56d5a5c1270bb152b0e6e0a
diff --git a/configure.ac b/configure.ac
index c186855..d82c78f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,6 +13,8 @@
 
 AC_CONFIG_TESTDIR(src/tests)
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])