Summary:
Fixes include:
1. Passing "GETDEPS_BUILD_DIR" and "GETDEPS_INSTALL_DIR" env variable and using them in eden/scm/Makefile rather than assuming the source code is always in the same place regardless getdeps arguments (it isn't).
2. Added "fbthrift-source" and "fb303-source" to avoid unnecessary compilation (at least of fb303) and to put fbthrift and fb303 source code in an easy to locate place inside getdeps' "installed" folder.

Pull Request resolved: https://github.com/facebookexperimental/eden/pull/25

Test Plan: sandcastle, check oss-eden_scm-darwin-getdeps

Reviewed By: farnz

Differential Revision: D22431872

Pulled By: lukaspiatkowski

fbshipit-source-id: 8ccbb090713ec085a5dd56df509eb58ab6fb9e34