Refactor CMake binding configuration

[rcurtin]

What is the desired addition or change?

This comes out of the discussion for the R bindings: #2556 (comment)

At the end of src/mlpack/CMakeLists.txt, there is a section for each binding type. It would be cleaner if we instead created macros like post_markdown_setup() for each of the existing binding languages (Julia, Go, Python) in each binding directory's CMakeLists.txt (i.e. src/mlpack/bindings/*/CMakeLists.txt), and then simply called those macros.

What is the motivation for this feature?

A cleaner CMake configuration. :)

If applicable, describe how this feature would be implemented.

So, in an ideal world, there would be a block at the end of src/mlpack/CMakeLists.txt like this:

post_python_setup()
post_julia_setup()
post_go_setup()
post_markdown_setup()

and that would be the only code in that file related to the bindings.

Additional information?

This is a pretty simple refactoring, so it could be a nice starter task for anyone looking to get involved with mlpack. Some CMake knowledge is probably necessary, but CMake isn't too hard to learn. :)

[rcurtin]

Thanks to @mikkac this was handled in #2608. 👍