Add Homebrew build incantation

(cherry picked from commit c0b595440714c26bc5b904e8844c1989662752fb)
We also require Homebrew on OSX

(cherry picked from commit 910903a30c24fe7cd9c03d6e32ef39dad0f2754d)

Conflicts:
	REQUIRE
Move XDG stuff from deps/build.jl to deps/ext_glib.jl

(cherry picked from commit b3d2af5d0f3319a3afe965c5c3f90699c6a303f2)
* 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

(cherry picked from commit 888cb58af15364734e21b0cc8ce78babc9361387)
split out __init__ into multiple functions for clarity, and so BinDeps doesn't override it

(cherry picked from commit 1c3350049caf0c2e28d9b1090a9a023f539de8c1)

Conflicts:
	src/GLib/signals.jl
unbreak linux build and cleanup library_dependency aliases to always list [unix-name, windows-name]