This adds support to compile the html python api docs from the command line by running `make doc_py` matching support between windows and unix.
This patch also makes it so the compiler is not needed if you set the `blender_bin` variable, this affects icon generation as well.
In the future, I want to move away from generating the build output in the build directory but that can come in a later change.
Reviewed By: LazyDodo
Differential Revision: https://developer.blender.org/D12144
