Makefile generator: fix relative paths to submakefiles
Makefile generator: fix relative paths to submakefiles

gyp can be ran from a directory other than the source root.
The relative paths we put into the root Makefile must be relative to
the source root.

Review URL: http://codereview.chromium.org/159661
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp