configure.ac: set -std=gnu11

Change-Id: I7d4c4e1f3663e448c5d044ccb4bbd3e813898f23
diff --git a/configure.ac b/configure.ac
index 44a7cff..b16acc8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6,6 +6,8 @@
 AM_INIT_AUTOMAKE([dist-bzip2 subdir-objects])
 LT_INIT([disable-static])
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 # kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])