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>
Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229