/bin/bash is not available on all systems

Use /bin/sh instead, and rewrite shell script to remove bashim.
Improve the portability of the tar rules

tar is a strange beast. Try to make a more portable rule.
The default on BSD is to write on /dev/rst0 while Linux seems to write
on stdout.

One other possible solution is
	tar czf - $* > $*.tar.gz
Simplify the packages rule

With the previous rule, gmake never matches properly the rules. Don't
know if it is a NetBSD issue or a gmake regression. Nevermind, I think
the new rules is correct enough.
Merge pull request #499 from adegroote/portability_tests

Improve the portability of code used in tests