Infrastructure for tig rpm builds

Infrastructure for tig rpm builds

Adds RPM_VERSION to Makefile and new make targets: tig.spec, dist, and
rpm. A simple 'make rpm' will build the rpm. Also adds tig.spec.in
which is used to generate tig.spec.

Accidentally VERSION (and adding -DVERSION=$(VERSION) to CFLAGS) is
now defined always, even if we do not compile from the "live" tig
repository.

Minimally tested (meaning "make rpm" from tig repository works).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>