Change USE_XORG to use the USES= framework.
Split up the old bsd.xorg.mk into two parts, Uses/xorg.mk and Uses/xorg-cat.mk.
Uses/xorg.mk contains the "user-facing" parts, related to how to depend on various xorg components. This relates to USE_XORG=
Uses/xorg-cat.mk contains the internal stuff, used by various xorg ports. This is for ports that set XORG_CAT=
Add legacy shims in bsd.port.mk to add USES=xorg or USES=xorg-cat if USE_XORG or XORG_CAT is defined.
Add two new (experimental) features to Uses/xorg-cat.mk:
Add provisions to Uses/xorg-cat.mk to use the meson build system instead of the autotools build system. This is done by specifying an optional build system argument to USES=xorg-cat.
Add provisions to Uses/xorg-cat.mk to pull sources straight from freedesktop.org gitlab (gitlab.freedesktop.org). This is mainly to make it easier for the development team. This can be used by adding USE_GITLAB=yes to an xorg port Makefile.
This change will eventually need an exp-run, but I'd like a review of the changes before that.
Branch with current work in progress can also be found here: https://github.com/FreeBSDDesktop/freebsd-ports/tree/feature/usesxorg