[RFC] Allow building as a static library

The first step to [refactoring Vim into a library](https://github.com/neovim/neovim/wiki/Refactor-vim-into-a-library) is to support compiling it as one.
As above, only add this definition to this target, I'm not entirely sure about the following snippet(untested)

    set_target_properties(nvim PROPERTIES COMPILE_FLAGS "${COMPILE_FLAGS} -DMAKE_LIB")