host/*/configure.ac: set -std=gnu11

Avoid using different dialects by accident (and resulting compiler
errors if compiler assumes a different dialect), like in [1].

Related: https://lists.osmocom.org/pipermail/openbsc/2019-September/013030.html
Related: https://lists.osmocom.org/pipermail/openbsc/2021-January/013360.html
Related: [1] libosmocore.git I72310886bef4db635078b75715c9d98ee45391cc
Change-Id: I48ec1703de04f40ea530f4b4442084fdc94dc966

src/host/gprsdecode/configure.ac

@@ -2,6 +2,8 @@ dnl Process this file with autoconf to produce a configure script
 AC_INIT([gprsdecode], [0.0.0])
 AM_INIT_AUTOMAKE
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 

src/host/layer23/configure.ac

@@ -2,6 +2,8 @@ dnl Process this file with autoconf to produce a configure script
 AC_INIT([layer23], [0.0.0])
 AM_INIT_AUTOMAKE
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 

src/host/osmocon/configure.ac

@@ -5,6 +5,8 @@ AC_INIT([osmocon],
 
 AM_INIT_AUTOMAKE([dist-bzip2])
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 

src/host/virt_phy/configure.ac

@@ -3,6 +3,8 @@ AC_INIT([virtphy], 0.0.0)
 AM_CONFIG_HEADER([config.h])
 AM_INIT_AUTOMAKE([foreign dist-bzip2 subdir-objects])
 
+CFLAGS="$CFLAGS -std=gnu11"
+
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])