waf: added --git-local-changes configure option

waf: added --git-local-changes configure option

if you use --git-local-changes then the version number that waf
extracts from git will have a '+' on the end if you have local
changes, as determined by running 'git diff'.

This used to be the default, but unfortunately it is far too slow on
some systems. On a NFS build system I was using the first line of
configure took about 2 minutes.

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Tue Nov 16 01:51:54 UTC 2010 on sn-devel-104