fix PythonLibs include

<PACKAGE>_FIND_VERSION seems to be unsupported in cmake 3.0. Also there is no
reason for this variable in this case anyway.
On Sat, Jun 21, 2014 at 05:28:25AM -0700, Dirk Thomas wrote:
> Shouldn't this require an exact version match then?
> 
>     find_package(PythonLibs "${PYTHON_VERSION_MAJOR}.${PYTHON_VERSION_MINOR}" EXACT REQUIRED)

That doesn't work over here at least.
That is because 2.7 != 2.7.7 and find_package detects
the latter one over here. If there is no extra flag available for that case,
I'd like to leave it that way.