explicitly disable expat, perl, python, and tcl/tk in git build
    
    We don't want any of the git code which requires expat, perl, python, or
    tcl/tk to be built.  Set the corresponding NO_* make variables.
    
    This ensures that even if git can find these tools in the buildroot, it
    won't use them.  For example, we have perl in the buildroot because it's
    used in the test suite, but we don't want to build any of git's perl
    tools.
    
    If nothing else, this should slightly improve the build time.