build: Add configuration summary

Meson now has a summary() function to easily summarize the build
configuration after the project was configured, use that for some
fancy output when the feature is available.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/249>

meson.build

@@ -101,3 +101,16 @@ meson.add_dist_script('meson/generate-stylesheets.py')
 meson.add_dist_script('meson/check-version.py',
   meson.project_version(),
   'NEWS')
+
+summary_options = {
+ 'extensions': enabled_extensions,
+ 'classic_mode': get_option('classic_mode'),
+}
+
+summary_dirs = {
+  'prefix': get_option('prefix'),
+  'datadir': get_option('datadir'),
+}
+
+summary(summary_dirs, section: 'Directories')
+summary(summary_options, section: 'Build Options')