configure.ac: set -std=gnu11

Avoid using different dialects by accident (and resulting compiler
errors if compiler assumes a different dialect), like in
I72310886bef4db635078b75715c9d98ee45391cc.

Related: https://lists.osmocom.org/pipermail/openbsc/2019-September/013030.html
Related: https://lists.osmocom.org/pipermail/openbsc/2021-January/013360.html
Related: osmo-pcu Ia57ba101627e3cc0babeca82631e207a3e2e0960
Change-Id: Id79b13d3c498acb565f91eba650328fccb5a13ef
diff --git a/configure.ac b/configure.ac
index 32ea7ca..2077570 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])])