meson: Fix plugin requirement checking and add logging

We were silently skipping plugins that didn't find a required feature,
even if the plugin option was enabled.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1604>

meson.build

@@ -394,8 +394,13 @@ endif
 
 foreach plugin_name, details: plugins
   plugin_opt = get_variable(f'@plugin_name@_option', get_option(plugin_name))
-  if plugin_opt.allowed()
+  if not plugin_opt.allowed()
+    debug(f'@plugin_name@ is disabled')
+    continue
+  endif
   plugin_deps_found = true
+
+  # Check whether we have all needed deps
   foreach dep_name, dep_ver: details.get('extra-deps', {})
     if dep_ver.length() != 0
       dep = dependency(dep_name, version: dep_ver, required: plugin_opt)
@@ -404,33 +409,43 @@ foreach plugin_name, details: plugins
     endif
     deps_cache += {dep_name: dep}
     if not dep.found()
+      debug(f'@plugin_name@ dependency @dep_name@ @dep_ver@ not found, skipping')
       plugin_deps_found = false
       break
     endif
   endforeach
-    plugin_features = details.get('features', [])
-    if plugin_deps_found
+  if not plugin_deps_found
+    continue
+  endif
+
   # Validate gst-plugin features
+  plugin_features = details.get('features', [])
   foreach feature: plugin_features
     if feature.startswith('gst-plugin') and not packages.contains(feature)
+      msg = f'@plugin_name@ required feature @feature@ not found'
+      if plugin_opt.enabled()
+        error(msg)
+      endif
+      message(msg + ', skipping')
       plugin_deps_found = false
       break
     endif
   endforeach
+  if not plugin_deps_found
+    continue
   endif
-    if details.has_key('gst-version')
+
   # Check if we have the required GStreamer version
-      gst_version = details.get('gst-version', '')
-      dep = dependency('gstreamer-1.0', required: false, version: gst_version)
-      if not dep.found()
-        opt = get_option(plugin_name)
-        if opt.enabled()
-          error('Required GStreamer version not found to build ' + plugin_name)
+  if details.has_key('gst-version') and not \
+      deps_cache['gstreamer-1.0'].version().version_compare(details['gst-version'])
+    msg = '@0@ requires gstreamer version @1@'.format(plugin_name, details['gst-version'])
+    if plugin_opt.enabled()
+      error(msg)
     endif
-        plugin_deps_found = false
+    message(msg + ', skipping')
+    continue
   endif
-    endif
-    if plugin_deps_found
+
   packages += f'gst-plugin-@plugin_name@'
   features += plugin_features
   extra_features = run_command('dependencies.py', meson.current_source_dir(), plugin_name,
@@ -450,8 +465,6 @@ foreach plugin_name, details: plugins
   if default_library in ['static', 'both']
     output += [lib + '.' + ext_static]
   endif
-    endif
-  endif
 endforeach
 
 feature_args = []