make: Fix a bug found by szager.
make: Fix a bug found by szager.

Previous output: ./gyp-mac-tool flock ?= LINK $(CXX)
Now: LINK ?= ./gyp-mac-tool flock $(CXX)
Review URL: http://codereview.chromium.org/8413060
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google