Refactor Makefile to remove all duplication

Created by: jonathonf

This is a fairly major refactor of the Makefile content to entirely remove duplication of commands, as well as ensuring all variable placeholders are populated or blanked appropriately (previously, placeholders were not always replaced and so the raw text was left in place).

It also corrects the use of a @PREFIX@ holder in the ms-office-online script where /usr/bin/env is instead "always" correct.