meson: build with _ISOC11_SOURCE on OpenBSD

Mesa builds with -std=c99 but uses timespec_get() a c11 function.
Build with _ISOC11_SOURCE for c11 visibility when -std is specified.
On linux c11 visibility comes from defining _GNU_SOURCE.

Fixes: e3a8013de8c ("util/u_queue: add util_queue_fence_wait_timeout")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5630>
1 file changed