Include Git revision in version info.

CMakeLists.txt contains now the version numbers for DMD and (next) LDC release.
If a .git folder is found then the LDC version is replaced by the first 6 chars
of the Git revision.

Possible improvements:
- If the build is not done at the master branch then it could be useful to
  check for a tag and use the tag instead of the revision. (for release builds)
- Maybe it is useful to include the branch name.

This fixes issue #366.