configure: check if compiler supports -Werror=vla.

Check if the compiler supports -Werror=vla before using it.
-Wvla was introduced with GCC 4.3 and is not present in 4.2.
Fixes the build on OpenBSD.

v2: Fix statement order, and quote $save_CFLAGS.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89433
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Jose Fonseca <jfonseca@vmware.com>
1 file changed