Update building.rst

Added additional instructions for successfully building in OS X
Update building.rst

-Added additional instructions for successfully building in OS X
-Fixed a link
Update building.rst

-Additional instructions for building in OS X
-Fixed typo
Update building.rst

Updated instructions for building 2.9.6.1 in Mac OS X
Merge pull request #17 from PoorDog/master

Additional instructions for building in OS X