Fix the make rule for re-running gyp to work if the script is in the root directory.
Fix the make rule for re-running gyp to work if the script
is in the root directory.

The problem was the make rule looked like:
Makefile: third_party/icu/icu.gyp ...
gyp_webkit -fmake --ignore-environment -Ibuild/common.gypi ...

But when it tried to run gyp_webkit, it complained that the command
wasn't found. After this patch, it will look like:
Makefile: third_party/icu/icu.gyp ...
./gyp_webkit -fmake --ignore-environment -Ibuild/common.gypi ...

Which should work if the file is executable.

[ submitting for tony@chromium.org - http://codereview.chromium.org/1733011 ]


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