build: Replace deprecated meson functions

Replace deprecated functions with their direct replacements:

 - dep.get_pkgconfig_variable() → dep.get_variable()
 - prg.path() → prg.full_path()
 - source/build_root() → project_source/build_root()

In one case we need meson.global_source_root() that was only
added in meson 0.58, so bump the requirement to that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2077>

js/dbusServices/meson.build

@@ -27,7 +27,7 @@ foreach service, dir : dbus_services
 
   serviceconf = configuration_data()
   serviceconf.set('service', service)
-  serviceconf.set('gjs', gjs.path())
+  serviceconf.set('gjs', gjs.full_path())
   serviceconf.set('pkgdatadir', pkgdatadir)
 
   configure_file(