Add git version.

Add git version.

git version is now printed in prox.log file.
- git_version.c is always recompiled
- If source have been locally changed, the build is shown as dirty
- If git is not available on the build system, a message such as
'make: git: Command not found'
will be printed, but the build will succeed.

Change-Id: If551832c6079f5e3bd04cd4d8ac5f8204e6be8cd
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>