Get configure.ac version values from git

Get configure.ac version values from git

Computes the VERSION, MAJOR_VERSION, MINOR_VERSION and MICRO_VERSION
values in configure.ac from 'git describe' output.

Adds new HASH_VERSION and GIT_VERSION, available as VERSION_GIT_HASH
and VERSION_GIT_FULL defines. On releases HASH_VERSION is empty and
GIT_VERSION is equal to VERSION (modulo EXTRA values). Full git version
is shown in about.

Requires latest release version to be tagged with an annotated tag.

Untested on win32 platforms, but should work as long as git and cut
binaries are on %PATH%.