Fixes examples/dmabuf-capture being built with unmet dependencies

Even if the libav* variables were disabler objects, the build targer
dmabuf-capture was being built. Modified the script to support a generic
solution.

examples/meson.build

@@ -20,31 +20,31 @@ endif
 examples = {
 	'simple': {
 		'src': 'simple.c',
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'pointer': {
 		'src': 'pointer.c',
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'touch': {
 		'src': ['touch.c', 'cat.c'],
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'tablet': {
 		'src': 'tablet.c',
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'rotation': {
 		'src': ['rotation.c', 'cat.c'],
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'multi-pointer': {
 		'src': 'multi-pointer.c',
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'output-layout': {
 		'src': ['output-layout.c', 'cat.c'],
-		'dep': wlroots,
+		'dep': [wlroots],
 	},
 	'screenshot': {
 		'src': 'screenshot.c',
@@ -93,10 +93,18 @@ examples = {
 }
 
 foreach name, info : examples
+	all_dep_found = true
+	foreach d : info.get('dep')
+		all_dep_found = all_dep_found and d.found()
+	endforeach
+	if all_dep_found
 		executable(
 			name,
 			info.get('src'),
 			dependencies: info.get('dep'),
 			build_by_default: get_option('examples'),
 		)
+	else
+		warning('Dependencies not satisfied for ' + name)
+	endif
 endforeach