Add definitions for DARWIN, clang and gcc

The definitions end up on the default help message of admsXml.
Use short GIT hash as revision number

Replace SVN version by GIT short hash.
The revision is shown by admsXml.
Add preprocessor definitions for Linux, GCC.

OS and compiler are shown by admsXml.
Merge pull request #7 from guitorri/admsXml-arch-compiler-git-stamp

Cosmetic changes to show git hash, architecture and compiler on admsXml call.