Makefile: make script-related rules usable from subdirectories

Makefile: make script-related rules usable from subdirectories

Git's Makefile provides a few nice features for script build and
installation (substitute the first line with the right path, hardcode the
path to Git library, ...).

The Makefile already knows how to process files outside the toplevel
directory with e.g.

  make SCRIPT_PERL=path/to/file.perl path/to/file

but we can make it simpler for callers by exposing build, install and
clean rules as .PHONY targets.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>