The way CMake's FindPythonLibs searches for versions is broken;
 'FIND EXACT' will fail if a different version is in Python_ADDITIONAL_VERSIONS
 and exists on the system, even if the desired version is also present.

We need Python_ADDITIONAL_VERSIONS to find recent Python versions when
 using old CMake, but FindPythonLibs implicitly adds the found version
 from FindPythonInterp to its search, so we can unset it after finding
 the interpreter.