build: simplify dep_egl a bit

Make sure dep_egl is always a valid dependency object, even if not
found. Ensure it is not found when not wanted, to avoid linking when
found but not wanted.

Using a not-found dependency in Meson is defined to be a safe no-op, so
use that to simplify the backend dependencies.

libweston/meson.build already errors out if renderer-gl is enabled and
EGL is not found, so the same checks can be removed from the backends.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/backend-drm/meson.build

@@ -33,6 +33,7 @@ srcs_drm = [
 ]
 
 deps_drm = [
+	dep_egl, # optional
 	dep_libm,
 	dep_libdl,
 	dep_libweston_private,
@@ -55,10 +56,6 @@ 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

libweston/backend-x11/meson.build

@@ -16,6 +16,7 @@ if not dep_x11_xcb.found()
 endif
 
 deps_x11 = [
+	dep_egl, # optional
 	dep_libweston_private,
 	dep_libdrm_headers,
 	dep_x11_xcb,
@@ -37,13 +38,6 @@ if dep_xcb_xkb.found()
 	config_h.set('HAVE_XCB_XKB', '1')
 endif
 
-if get_option('renderer-gl')
-	if not dep_egl.found()
-		error('x11-backend + gl-renderer requires egl which was not found. Or, you can use \'-Dbackend-x11=false\' or \'-Drenderer-gl=false\'.')
-	endif
-	deps_x11 += dep_egl
-endif
-
 plugin_x11 = shared_library(
 	'x11-backend',
 	srcs_x11,

libweston/meson.build

@@ -81,6 +81,8 @@ if get_option('renderer-gl')
 		error('libweston + gl-renderer requires egl which was not found. Or, you can use \'-Drenderer-gl=false\'.')
 	endif
 	deps_libweston += dep_egl
+else
+	dep_egl = dependency('', required: false)
 endif
 
 if get_option('backend-vnc')