Prevent CMake Error on Custom Python Builds

[mattdawkins]

Without this change, on custom python builds (for example, the debug CPython build I just did on windows) the sys.prefix variable can sometimes be empty in the above python call, leading to only 5 arguments being passed into the cmake regex call here, result in a cmake configure time error. Adding quotes ensures 6 arguments are passed in.

[mattdawkins]

CMake Error at tools/FindPythonLibsNew.cmake:137 (string):
  string sub-command REGEX, mode REPLACE needs at least 6 arguments total to
  command.
Call Stack (most recent call first):
  tools/pybind11Tools.cmake:16 (find_package)
  CMakeLists.txt:33 (include)

[wjakob]

LGTM, thanks!