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: I76ed877d160f98e3627293b0539bec5fac1e8469
---
M configure.ac
1 file changed, 18 insertions(+), 0 deletions(-)

diff --git a/configure.ac b/configure.ac
index 1f4330b..65ebd59 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])])
 

