configure.ac: set -std=gnu11

Change-Id: Ib920af17b9a0b5d6ea5f4db33faab3a2dcf8fbbe
diff --git a/configure.ac b/configure.ac
index a7dde2f..6a3bc4b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -8,6 +8,8 @@
 AM_INIT_AUTOMAKE([foreign dist-bzip2 no-dist-gzip 1.6 subdir-objects])
 AC_CONFIG_TESTDIR(tests)
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])