build: Yield 'man' option to extension-tool subproject

Turns out meson has a build-in pattern of what we are doing, namely:
Set a subproject option to a parent project option of the same name.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1072

meson.build

@@ -246,7 +246,6 @@ libgvc_gir = libgvc.get_variable('libgvc_gir')
 if get_option('extensions_tool')
   extension_tool = subproject('extensions-tool',
     default_options: [
-      'man=@0@'.format(get_option('man')),
       'package_name=@0@'.format(meson.project_name()),
     ]
   )

subprojects/extensions-tool/meson_options.txt

@@ -1,7 +1,8 @@
 option('man',
   type: 'boolean',
   value: true,
-  description: 'Generate man pages'
+  description: 'Generate man pages',
+  yield: true,
 )
 
 option('package_name',