Check the egl, glesv2 and gbm dependencies locally instead of relying on
the dep_* variables from the top level meson.build or
libweston/meson.build (dep_gbm).

This should make these dependencies now explicitly checked when the app
is built, rather than relying on other components doing the checks. If
the drm-backend was disabled, this would have probably hit an error
using the undeclared variable dep_gbm.

Signed-off-by: Pekka Paalanen <pq@iki.fi>