Figure out how to include dependency source code in a distribution and package it up

[theory]

Experiment with adding the source for a dependency to a source code distribution such that it builds, installs, and can be packaged transparently.

Maybe try with pgsql-http. Fork it, and add the source to build the libcurl dynamic library to the project (directly, via a Git submodule, or by Makefile target) and update its Makefile to get it working with make install && make installcheck.

This pattern is probably already pretty well-supported by PGXS: should be able to something like add the library's .c file to MODULES and see it installed in the package lib dir alongside the extension module itself.

Then make sure it works for the packaging patterns developed in #52.

Done with the pattern is proven to work, ideally one example lives in the #45 registry, and the pattern has been well-documented.