Fix pathname relativisation of variable defaults.

Variables set as defaults with the "foo%" syntax were not being
relativized because the % was being treated as part of the name and thus
the _path suffix was not seen. Add % to the set of characters stripped
from the name in IsPathSection.

BUG=
R=mark@chromium.org

Review URL: https://codereview.chromium.org/15675013

git-svn-id: http://gyp.googlecode.com/svn/trunk@1637 78cadc50-ecff-11dd-a971-7dbc132099af