build: ask for -std=gnu99 explicitly

build: ask for -std=gnu99 explicitly

We previously got -std=gnu99 secretly enabled as a side-effect
of requesting the 'stdarg' gnulib module. We rely on some
extensions from c99/gnu99 and while RHEL-7 supports this, it
still defaults to gnu89.  RHEL-7 also supports some newer
standards but declares them experimental/incomplete, so sticking
with gnu99 is best bet for now & matches historical usage.

Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>