sync-and-gyp: make smarter

sync-and-gyp: make smarter

This fixes two issues: If gyp is run with the CXX and CC environment
variables set, it will produce different ninja build files.
sync-and-gyp now tracks the values of those variables; if they change,
re-run gyp.

Secondly, we introduced find.py, which is used inside gyp files.  If
the contents of certain directories changes, gyp should be re-run.  So
now we track those directories too.

Review URL: https://codereview.chromium.org/1153083002