use standard `__typeof__()` instead of GNU extension `typeof()`

And switch to c_std=c99. This simplifies using libdrm as a meson
subproject for mesa.

v2: (dylan)
  - switch to c99 as the standard
  - Fix amdgpu security tests as well

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Emma Anholt <emma@anholt.net>