Abstract Out the Program Used for 'makeprg'

*Issues Referenced*: [#26][], [#30][].

This would pull out the use of direct `make` commands and allow cmake.vim 
to intelligently determine which build toolchain like `GNU make` or `ninja` to use.

[#26]: https://github.com/jalcine/cmake.vim/issues/26
[#30]: https://github.com/jalcine/cmake.vim/issues/30