In preparation of the update of glib remove the -reference ports

Those ports mainly concern old Gnome2 libraries, the behaviour of this infra
is not compatible with the meson build system (being used in newer version)
the documentation is provided otherwise in the other version

Remove ${PORTSDIR}/ from dependencies, categories r, s, t, and u.

With hat:	portmgr
Sponsored by:	Absolight

Convert GMAKE to MAKE_CMD
Please note that lots of invocation of MAKE_CMD here are wrong as they do not
properly respect MAKE_ENV and friends

With hat:	portmgr

- Stage support for all -reference ports
- Nuke NOPORTDOCS, those ports are already IGNOREd with NOPORTDOCS

Approved by:	kwm (maintainer)

Add NO_STAGE all over the place in preparation for the staging support (cat:
textproc)

Fix build dependency line

Feature safe:	yes

Attempt to fix the NOPORTDOCS case.

Reported by:    QAT

- Add header in these *-reference ports.
- Fix a few of pkg-descr by chase the rename.
- Move all PORTREVISION and PORTEPOCH to top with ?=.
- Put USE_X_PREFIX back in, but under REFERENCE_PORT, and remove PREFIX? and
  USE_XLIB. This fix ports to use the correct mtree when you change the prefix,
  for example:

        Incorrect: (Without USE_X_PREFIX)
        ================================
        # cd /usr/ports/x11-toolkits/gtk20
        # make -V MTREE_FILE
        /etc/mtree/BSD.x11-4.dist
        # make PREFIX=/tmp/foo -V MTREE_FILE
        /etc/mtree/BSD.local.dist <-- Here...
        ================================

- Add pkg-descr

Requested by:   Dan Langille <dan@langille.org> for freshports.org

Move the libxslt programming reference to the libxslt-reference port.