configure.ac: set -std=gnu11

Change-Id: I7fed7d43242f804e6d2b005277c5b2b1bd197aa8
diff --git a/configure.ac b/configure.ac
index 9d8b4bf..a1c7180 100644
--- a/configure.ac
+++ b/configure.ac
@@ -9,6 +9,8 @@
 AC_CONFIG_TESTDIR(tests)
 AC_CANONICAL_HOST
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])