generator output should be relative to depth.

Currently, it's a strange mix between depth, the filename, and generator
output that happens to work if depth is .

BUG=none
TEST=test/generator-output/gyptest-depth.py
R=joi@chromium.org

Review URL: https://codereview.chromium.org/127893002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1830 78cadc50-ecff-11dd-a971-7dbc132099af