Support installing from a makefile

Nobody has claimed this yet.

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](https://github.com/influxdata/flux/issues/2735).

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.

Start by inspecting the current pkg-config module download, temporary install-directory, and pkg-config-path handling. Compare that flow with the requested make build and make install steps in the package directory, then resolve where cross-compilation belongs. Done means a module-local makefile can build and install into the temporary prefix while its generated pkgconfig directories are discovered.

Written by the indexing model from the issue text.