libweston/backend/drm: might need EGL

gbm-drm.c includes gl-renderer.h. When EGL is enabled, that in turns
includes egl.h. As such, dependencies for drm should include EGL if
it is available.

This condition is modelled after a similar one in libweston/meson.build

Reported-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reported-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Refik Tuzakli <tuzakli.refik@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>

libweston/backend-drm/meson.build

@@ -53,6 +53,10 @@ if get_option('renderer-gl')
 		config_h.set('HAVE_GBM_FD_IMPORT', '1')
 	endif
 	deps_drm += dep_gbm
+	if not dep_egl.found()
+		error('drm-backend + gl-renderer requires egl which was not found. Or, you can use \'-Dbackend-drm=false\' or \'-Drenderer-gl=false\'.')
+	endif
+	deps_drm += dep_egl
 	srcs_drm += 'drm-gbm.c'
 	config_h.set('BUILD_DRM_GBM', '1')
 endif