configure.ac: set -std=gnu11

Change-Id: I708fe349fd506613af0e62a62bf6cbe611a06eef
diff --git a/configure.ac b/configure.ac
index 4682931..63615b8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -8,6 +8,8 @@
 
 AM_INIT_AUTOMAKE([dist-bzip2 foreign])
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])