CMake: Remember Python Version (pybind#1434)

It is useful not only to remember the python libs and includes but
also the interpreter version in cache.

If users call pybind11 throught `add_subdirectories` they will
otherwise have no access to the selected interpreter version.
The interpreter version is useful for downstream projects, e.g.
to select default `lib/pythonX.Y/site-packages/` install paths.

CMakeLists.txt

@@ -38,6 +38,8 @@ set(PYTHON_INCLUDE_DIRS ${PYTHON_INCLUDE_DIRS} CACHE INTERNAL "")
 set(PYTHON_LIBRARIES ${PYTHON_LIBRARIES} CACHE INTERNAL "")
 set(PYTHON_MODULE_PREFIX ${PYTHON_MODULE_PREFIX} CACHE INTERNAL "")
 set(PYTHON_MODULE_EXTENSION ${PYTHON_MODULE_EXTENSION} CACHE INTERNAL "")
+set(PYTHON_VERSION_MAJOR ${PYTHON_VERSION_MAJOR} CACHE INTERNAL "")
+set(PYTHON_VERSION_MINOR ${PYTHON_VERSION_MINOR} CACHE INTERNAL "")
 
 # NB: when adding a header don't forget to also add it to setup.py
 set(PYBIND11_HEADERS