Specially because I am in Mac and I might miss most of the needed tools :-/
Depending on how serious you are to help with Lilypond Dev, I was also
on a mac for 2 years helping but I used VirtualBox and the pre-built
Linux Environment as a Virtual Machine - LilyDev - which you can find
all about here:
http://lilypond.org/doc/v2.15/Documentation/contributor/lilydev
Actually I still use LilyDev in VirtualBox, but on a Linux Machine :)
I prefer to keep my LilyPond Dev environment separate from my real
machine (as I can also use snapshots in VirtualBox so if I make a huge
mistake I can roll back in seconds).