Revert "Add GLIB version guards"

This reverts commit c0c9fea52df5a0bf7a34805d1917747716c678cb.

Setting GLIB_VERSION_MAX_ALLOWED prevents building on machines with more recent
GLib versions for which GLIB_VERSION_MIN_REQUIRED may than be lower than
GLIB_VERSION_MAX_ALLOWED. That is strictly forbidden by GLib itself.

As a workaround, the definition can and should be passed on the command line.