Fix null-pointer dereference during well construction

The three-argument WellsManager constructor needs access to the real
permeability field lest a null-pointer dereference result when the
problem actually contains any wells.
Instructions on how to build prerequisites on MacOS X

Although it is possible using the GNU toolchain, there is
probably a larger base which has the Apple toolchain and want to
use that.
Remove unused "typedef"s

CLang and recent versions of GCC warn about unused "typedef"s.  This
change-set removes currently known instances in opm-core.
Merge pull request #399 from rolk/399_macosx

Add instructions on how to build prerequisites on MacOS X
Merge pull request #400 from bska/Wunused

Remove unused "typedef"s
Merge pull request #395 from bska/nullptr-deref

Fix null-pointer dereference during well construction