ninja: Add an opt-in option for the new deps feature.

This is supposed to be used for letting a couple of folks opt in to this new
feature (which requires a ninja trunk build), to help stabilize deps mode in
ninja. Once there's a stable ninja release with the feature (and ninja in
depot_tools was updated), this will be turned on by default.

Use this by passing -Guse_deps=1 to ninja (or adding use_deps=1 to the
GYP_GENERATOR_FLAGS env var). It's off by default.

deps mode on windows needs more work, so this opt-in is linux/mac only for
now.
Review URL: https://codereview.chromium.org/14180006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1610 78cadc50-ecff-11dd-a971-7dbc132099af