configure.ac: set -std=gnu11

Change-Id: I52c86f461edd96d810fb85a60c68be581caa1799
diff --git a/configure.ac b/configure.ac
index 111d477..6660bf1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6,6 +6,8 @@
 AM_INIT_AUTOMAKE([dist-bzip2])
 AC_CONFIG_TESTDIR(tests)
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])