build: Use XDT_CHECK_PACKAGE
This allows in particular to set GLib macros when building with xfce4-dev-tools >= 4.17.1, which shows that GLib >= 2.38 is required.
This allows in particular to set GLib macros when building with xfce4-dev-tools >= 4.17.1, which shows that GLib >= 2.38 is required.