Add Homebrew build incantation
We also require Homebrew on OSX
Move XDG stuff from deps/build.jl to deps/ext_glib.jl
* Update to use new BinDeps features, namely library groups and onload hooks!
* Add a .gitignore to never commit deps/deps.jl, along with those pesky .DS_Store files
Merge branch 'sf/homebrew'
split out __init__ into multiple functions for clarity, and so BinDeps doesn't override it
unbreak linux build and cleanup library_dependency aliases to always list [unix-name, windows-name]