meson: Add support for configuring dri drivers directory.

v2: - drop with_ from dri_drivers_path variable (Eric A)
v3: - Move HAVE_X11_PLATFORM to the proper patch (Eric A)

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
4 files changed