Support a generator flag output_dir to control the name of the build dir; defaults to "out" still.
Support a generator flag output_dir to control the name of the build dir; defaults to "out" still.

Allows '-G output_dir=sconsbuild' to have the output dropped where a bunch of scripts expect.  (you can also use GYP_GENERATOR_FLAGS in the env)
Review URL: http://codereview.chromium.org/152002
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp