Put the git version into all executables.

Put the git version into all executables.

This patch changes the -V output of all executables to print also
the git version which was used to make the executable. The idea and
the GIT-VERSION_GEN script was taken from git.git.

It also adds a VERSION file to the generated tarball. If this file
exists, its content is used as the version info rather than calling
git to obtain this info. This way, even if paraslash is built from
a tarball, the executables still contain the git version the tarball
was generated from.