Regression fix for 3cd622cf.

 Fixes ArcticaProject/nx-libs#890.

Make nxproxy and nxagent print an arbitrary version number by running
make NX_VERSION_CUSTOM="myvers". nxagent and nxproxy will then show
this in addition to the original version number:

NXPROXY - Version myvers (3.5.99.22)
NXAGENT - Version myvers (3.5.99.22)

Fixes ArcticaProject/nx-libs#606

This will help on non-supported platforms as it will add a path the current
code might not be aware of.

Makefile: Add clean/distclean target calls for nxdialog subdir to master Makefile's clean/distclean targets.

ensure that we have all the required def files

Regression fix for a93f64e4. The test executables bails out on test failure, whereas ifdef does not. Adding "|| true" where needed.

create a new def file nxconfig.def and be fine. The settings in that
file will be respected by all subsequent imake calls anywhere in the tree.

(Very) old SLE versions don't have that file at all and we can do
without - we'll just use the previous, default values.

This also means that we don't need to preinstall a package containing
xkbcomp.pc on Debian and derivates for the cleaning target.

Fixes: ArcticaProject/nx-libs#708

Makefile targets: Rewriting and consolidating the target structure for nx-X11/ subdir (except nx-X11/lib/).

Makefile: Don't do a parallel clean-up of nx-X11. We have seen failures and due to the upcoming switch to autotools, it is pointless to dive into fixing it in the Imakefile stack.

Legacy Xext proto versions are automatically detected by the main
Makefile.

Avoids errors like these (and of course also is a good idea in general):
[   36s] make -C nx-X11 BuildIncludes FONT_DEFINES= -DLEGACY_XFONT1
[   36s] make: invalid option -- D
[   36s] make: invalid option -- E
[   36s] make: invalid option -- G
[   36s] make: invalid option -- A
[   36s] Usage: make [options] [target] ...

Partly reverts ec30a857, but in a
forward-compatible manner.

Legacy libXfont1 versions are automatically detected by the main
Makefile.

 Fixes ArcticaProject/nx-libs#547.

 Fixes ArcticaProject/nx-libs#546.

 This also solves the last remnant of overlinking as described in GH issue #133.

 Fixes ArcticaProject/nx-libs#133.

git-subtree-dir: nx-X11/extras/Mesa_6.4.2
git-subtree-split: 475b1f7b56fa36ef4f3b22a749f88a98ddc0d502

git-subtree-dir: nx-X11/extras/Mesa_6.4.1
git-subtree-split: 53d1bc081eb633aecbd5ab0bf3981f734a2102f2

When descending subdirectories and calling configure pass
`--prefix=${DESTDIR}${PREFIX}`, so that installing in a
custom directory works.

Makefile: Make sure that 'make all' only calls 'make build' and _not_ the global '%:' ruleset afterwards.