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>

subprojects/extensions-app/js/meson.build

@@ -9,7 +9,7 @@ endif
 launcherconf.set('prefix', prefix)
 launcherconf.set('libdir', libdir)
 launcherconf.set('pkgdatadir', pkgdatadir)
-launcherconf.set('gjs', gjs.path())
+launcherconf.set('gjs', gjs.full_path())
 
 configure_file(
   input: prgname + '.in',