This project provides an example of using ExternalBuild

The idea is that some external project (in this case libpng) already provides it's own build system. We don't want to rely on a prebuilt package (e.g. via pkg-config), but instead want to build, link and install build products from that external project.

Here we build and link pngtest.c against the libpng static library (shared libraries not yet supported)

We also provide a phony target 'fetch' that fetches the source code for libpng into our tree, for example, if you don't want to use git submodules.