Writing to the python helper script moved to `python/volk_modtool/CMakeLists.txt`

…n pull req.

This reverts commit d4afa9e partially.

as @jdemel stated:
> CMAKE_CURRENT_BINARY_DIR would possibly be the preferred way to use. I'd suspect changing this variable in the original location would already fix your issue. That'd be great. That way we can keep the code in a more generic place that makes it easier to re-use. Also, thanks for your patience. 

I suspected about is there any upper config deletes CMAKE_CURRENT_SOURCE_DIR and CMAKE_CURRENT_BUILD_DIR and it overriding on 'CMakeLists.txt'.