Support installing from a makefile

[Jonathan A. Sternberg]

The pkg-config utility should be modified to support building from a makefile that is located in the same directory as the package it is building.

This is meant to be a replacement to the current build code for a specific module as detailed here.

When pkg-config executes inside of a module, it will perform the logic to download the module and ensure it is writeable. Then it will change to the package directory and run make build; make install and it will create a temporary install directory like it currently does. It will override the prefix for install into that temporary directory and then it will add any created pkgconfig directories to the pkg-config path just like it currently does.

The cross compilation logic will either live as part of the makefile for the individual project or we'll somehow incorporate the cross-compilation logic into pkg-config. Whichever makes more sense.