ci: Add jobs to test -std=c99 builds.

It should generally be possible to build OVS with mostly c99 standard
and without GNU extensions.

It's not a pure c99 build, because some non-standard features from
POSIX variants are always enabled if available via
AC_USE_SYSTEM_EXTENSIONS.  Without them we'll not have some essentials
like RW locks, for example.

'-std=c99' doesn't disable all the GNU extensions, only some of them.
It's technically possible to use '-fgnuc-version=0' with clang to
fully disable all the extensions, but it is causing build assertions
with the version of clang provided in Ubuntu 20.04 in GHA (some
issue with packed structures not being correctly packed).  We may
use this option in the future after the base image upgrade.

Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

.ci/linux-build.sh

@@ -82,6 +82,10 @@ if [ "$DPDK" ] || [ "$DPDK_SHARED" ]; then
     install_dpdk
 fi
 
+if [ "$STD" ]; then
+    CFLAGS_FOR_OVS="${CFLAGS_FOR_OVS} -std=$STD"
+fi
+
 if [ "$CC" = "clang" ]; then
     CFLAGS_FOR_OVS="${CFLAGS_FOR_OVS} -Wno-error=unused-command-line-argument"
 elif [ "$M32" ]; then

.github/workflows/build-and-test.yml

@@ -85,6 +85,7 @@ jobs:
       LIBS:        ${{ matrix.libs }}
       M32:         ${{ matrix.m32 }}
       OPTS:        ${{ matrix.opts }}
+      STD:         ${{ matrix.std }}
       TESTSUITE:   ${{ matrix.testsuite }}
 
     name: linux ${{ join(matrix.*, ' ') }}
@@ -100,6 +101,11 @@ jobs:
           - compiler:     clang
             opts:         --disable-ssl
 
+          - compiler:     gcc
+            std:          c99
+          - compiler:     clang
+            std:          c99
+
           - compiler:     gcc
             testsuite:    test
           - compiler:     clang