Add support for the syzygy.gyp_env files.

The file should be located in src/ (next to DEPS.syzygy) and called syzygy.gyp_env. It allows to overwrite some gyp variables.

This is pretty much a copy paste from https://code.google.com/p/chromium/codesearch#chromium/src/build/gyp_helper.py&l=14
BUG=
R=erikwright@chromium.org

Review URL: https://codereview.appspot.com/157060043