Fix and refactor substituting include/gpb_version.hrl

Refactor the substitution into a separate auxiliary shell script:
build/mk_version_hrl. This made it easier to get working
under both Linux Mac OS X.

Also, the substitutions were previously changed to work also
for BSD sed, for instance on Mac OS X, but the substituted
text lines got swapped. Now fixed to work with both a GNU
and a BSD sed.
Add a Makefile

This is as a complement to the rebar build setup.