Added pre-make command for deps

Some dependencies (like github.com/omerk/pihwm) does not have a Makefile - they use things like autotools to generate the Makefile.

By adding a 3rd argument to the dep_ line it is now possible to run a command prior to executing make.

Example:
dep_pihwm = https://github.com/omerk/pihwm.git master "./configure"

Tested on the Raspberry Pi where it works.