meson: Add a GBM backends search path build option

This is the list of locations where GBM will look
for alternate backend implementations based on the
DRM driver name. It defaults to $libdir/gbm.

On Android, this path is currently hard-coded to
/vendor/lib[64]/gbm.

Signed-off-by: James Jones <jajones@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9902>

meson_options.txt

@@ -270,6 +270,12 @@ option(
   choices : ['auto', 'true', 'false', 'enabled', 'disabled'],
   description : 'Build support for gbm platform'
 )
+option(
+  'gbm-backends-path',
+  type : 'string',
+  value : '',
+  description : 'Locations to search for gbm backends, passed as colon separated list. Default: $libdir/gbm.'
+)
 option(
   'glx',
   type : 'combo',