Fix #149: support for OS X users who have Xcode / Developer tools installed

add some more information on how to make sure the correct version of crags is used - especially if user has Xcode / Developer tools installed.
Merge pull request #162 from tkothe/patch-1

Fix #149: support for OS X users who have Xcode / Developer tools instal...
Merge branch 'master' into python3-update

Conflicts:
	README.creole