Throw error on missing packages

[iloveitaly]

As I mentioned here the ASDF_PKG_MISSING variable doesn't actually do anything. This change actually throws an error if a package is missing.

Also added a link to a blog post where I detail this out further.

[smorimoto]

This does not work except on macOS.

[iloveitaly]

@smorimoto looks like the non-macos script wasn't building with libjpeg/libpng at all. I've fixed this and the build is working on all platforms.