Patch cmake to allow detection of python 2.7.9+

Some distributions of python have their version defined as follows in patchlevel.h (note the '+'):
#define PY_VERSION "2.7.9+"
The '+' char needs to be stripped by the cmake regex so that LLDBs python lib detection is successful.

Differential Revision: http://reviews.llvm.org/D15566

llvm-svn: 255893