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>

docs/reference/shell/meson.build

@@ -41,8 +41,8 @@ configure_file(
 gnome.gtkdoc('shell',
   main_sgml: 'shell-docs.sgml',
   src_dir: [
-    join_paths(meson.source_root(), 'src'),
-    join_paths(meson.build_root(), 'src')
+    join_paths(meson.project_source_root(), 'src'),
+    join_paths(meson.project_build_root(), 'src')
   ],
   scan_args: [
     '--ignore-headers=' + ' '.join(private_headers + exclude_directories)