xorg/lib stuff to enable mesa.old, prepare for new mesa

- rename OLD_PREFIX to OLD_SUFFIX, since it is
- include mesa-which.mk and use ${OLD_SUFFIX} in several places
- remove mesa < 18 support

move mesa10 build into .old scheme:

- add ${X11SRCDIR.MesaLib.old} and introduce ${X11SRCDIR.Mesa} as an
alias for either the former or the non old. this allows many of
the makefiles to simply

move mesa10 build into .old scheme:

- add ${X11SRCDIR.MesaLib.old} and introduce ${X11SRCDIR.Mesa} as an
alias for either the former or the non old. this allows many of
the makefiles to simply use ${X11SRCDIR.Mesa} (but does not really
enable much sharing of makefiles, but reduces their diffs.)

- use mesa-which.mk to define ${OLD_PREFIX} to either "" or ".old",
and to know if to build 'dri7' (.old only.) ${OLD_PREFIX} is used
by other code (eg, LIBDPLIBS) to pick the right subdir.

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update sets and reachover makefiles for mesa 10.3.5.

New variable MKX11MOTIF to enable building libGLw with Motif support.
Defaults to 'no'.

Install pkgconfig files. Where applicable, remove .if 0 code that tried to
do the same thing.

initial import of the crossbuild framework for modular Xorg. this
is heavily based upon the work rtr@ did on src/x11, but hacked to
work for modern modular Xorg source split.

the sources will live

initial import of the crossbuild framework for modular Xorg. this
is heavily based upon the work rtr@ did on src/x11, but hacked to
work for modern modular Xorg source split.

the sources will live in xsrc/external/mit/<package>/dist.

documentation on using this will come when the above directories
are fully populated. there are 192 of them so it may take some time
before that is completed.

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