Que Quotion authored Nov 27, 2018 and Allan McRae committed Nov 27, 2018

This opens the door for third parties to provide libmakepkg
extentions for the purpose of altering the build environment.

Signed-off-by: Que Quotion <quequotion@gmail.com>
Signed-off-by: Allan McRae <allan@archlinux.org>