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

This opens the door for third parties who provide extensions to
libmakepkg to supply scripts that confirm the presence of their
dependant executables.

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