bldchn, import git-version-gen from git sources v1.8.2-61-g7b9a419

The original name was GIT-VERSION-GEN.
bldchn, change git-version-gen to what we need

- set correct DEF_VER
- usage: git-version-gen [srcdir [builddir/version.mk]]
- version.mk sets VERSION, PACKAGE_VERSION and PACKAGE_STRING
- make "obs-snap" style version numbers, e.g. "0.4.0.git32.f350f"
- safe the version files timestamp (when building from dist tarball)
- nicer info update message
- fix portability glitches, quoting, backticks, etc.

We still should review the style, error handling and maybe add some
comments.
bldchn, add git-version-gen to the dist
bldchn, never use AC_INIT's version info
bldchn, initially generate version.mk at configure time

... adding a new macro GIT-VERSION-FILE.
bldchn, all Makefiles include include version.mk
bldchn, add GNUmakefile which should do nothing so far
bldchn, GNUmakefile updates version.mk at make time
bldchn, dist-hook creates distdir/version file
bldchn, add do_subst function

We want to use this in future instead of AC_CONFIG_FILES to
enrich some files with an up-to-date VERSION.
bldchn, create spec file at make time
bldchn, create .pc pkg-config file at make time
bldchn, avoid gnuism for do_subst rules
bldchn, remove version info from config header ...

... just to see who is using it.
build version_defs.h and version.o
use the new version.h
bldchn, gengetopt files do not need VERSION
bldchn, man pages depend on VERSION
Merge branch 'bld/git-version-gen'

* bld/git-version-gen:
  bldchn, man pages depend on VERSION
  bldchn, gengetopt files do not need VERSION
  use the new version.h
  build version_defs.h and version.o
  bldchn, remove version info from config header ...
  bldchn, avoid gnuism for do_subst rules
  bldchn, create .pc pkg-config file at make time
  bldchn, create spec file at make time
  bldchn, add do_subst function
  bldchn, dist-hook creates distdir/version file
  bldchn, GNUmakefile updates version.mk at make time
  bldchn, add GNUmakefile which should do nothing so far
  bldchn, all Makefiles include include version.mk
  bldchn, initially generate version.mk at configure time
  bldchn, never use AC_INIT's version info
  bldchn, add git-version-gen to the dist
  bldchn, change git-version-gen to what we need
  bldchn, import git-version-gen from git sources v1.8.2-61-g7b9a419