Prevent passed in CFLAGS and friends from trashing gyp-configured flags.
Prevent passed in CFLAGS and friends from trashing gyp-configured flags.

Patch (slightly modified) from fta@sofaraway.org
BUG=http://code.google.com/p/gyp/issues/detail?id=123
Review URL: http://codereview.chromium.org/500033

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp