configure.ac: set -std=gnu11

Change-Id: Ic2a1f20be65810a86cdc36aaa459819146a0bfcc
diff --git a/configure.ac b/configure.ac
index 83885d4..85b46eb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -9,6 +9,8 @@
 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])])