Patch to add GIT support

The attached patch to the build system adds the ability for version
stamping to happen either when working in an SVN repo or GIT repo. In
an SVN repo, it will stamp the repo version number, such as 2134:2137M
as the current code does. In a GIT repo, it'll stamp the short version
of the last commit hash - eg. 42c03da.

Thanks to some^Wa lot of help from Robert Haas, it also manages to fix
the old problem in which the build system always thought that the
svnversion.h header had changed, thus resulting in svnversion.cpp
being recompiled and everything being relinked, regardless of whether
anything actually did change.

I need to do something similar for VC++, but does anyone object to
this? The intention is to apply it to all important branches so when
we move to GIT, they'll just work.

--
Dave Page
EnterpriseDB UK: http://www.enterprisedb.com
The Enterprise Postgres Company