build.jl files should include dependencies

[staticfloat]

Now that building dependent packages is working pretty well (see, for instance, GlibBuilder) we should make sure that the generated build.jl files chain up with the other build.jl files automatically, so that if the user downloads the auto-generated build.jl file, it will automatically fetch and install all necessary dependencies.

[staticfloat]

So far we have the unofficial agreement that, if binary package FLAC requires binary package Ogg, then this implies that FLAC.jl also requires Ogg.jl, meaning that within FLAC.jl there is a using Ogg. This causes Ogg.jl to dlopen("libogg"), which then causes a dlopen("libFLAC") to not fail when it tries to lookup libogg as libogg is already loaded into the julia process.

However, we now have situations where this isn't the case; we need a way for a build.jl to signify that it might need to install another binary package. I'm comfortable with having FLAC.jl cause the installation of Ogg.jl, but we shouldn't require FLAC.jl to run using Ogg within itself. What would be ideal is for FLAC.jl/deps/build.jl to express a dependence on Ogg.jl/deps/build.jl, and automagically extract the products from that build.jl, and find them at Pkg.build() time, so that when the user runs using FLAC, which then runs include("deps/deps.jl"), the auto-generated deps.jl can dlopen(libogg_path) and then dlopen(libflac_path).

In order for this to work, we need a few things:

• We need to be able to extract the products from another package. I'm thinking this could be as simple as just include(depsjl_path) within our build.jl.

• We need for build.jl files to express a concept of "dependencies". At build_tarball.jl time, it is appropriate to encode this as an URL to a build.jl file. At Pkg.build() time, I argue this is not appropriate and we should assume that Pkg has done its job and everything we need is already installed. Ideally, I'd like an encoding that looks like:

dependencies = [
    Pkg.dir("Ogg", "deps", "deps.jl") => [:libogg],
    Pkg.dir("CodecZlib", "deps", "deps.jl") => [:libz],
]

This would express the idea that we need libogg.so from the Ogg package and libz from the CodecZlib package to be loaded before we can load our own libraries. This dependencies would be passed in to install(), and the generated deps.jl file would read in the dependent deps.jl files, locate the paths proffered by the libogg and libz variables defined within those deps.jl files, and finally dlopen() them before we attempted to dlopen() our own libraries.

It is unfortunate that there's no way for us to magically go from OggBuilder URL to the package Ogg, so this is a case where I think we're going to have to ask the user to provide more information manually. :(

[staticfloat]

This is fixed by the implicit dependency web that JLL packages create. Huzzah!