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
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp