what does this line? If we have `depends_on 'samtools'`, it should not be needed. Why are we loading it here?
We really should try to make it work (and therefore perhaps update/alter the samtools formula) with the existing formula.

Often there is a configure or a variable that we can pass. Or at least we could `inreplace` the makefile, so that it finds samtools.
This might be useful:
`Formula.factory('samtools').opt_prefix/'lib'` or ``Formula.factory('samtools').opt_prefix/'include'`