Fix incorrect depfile path relativization

While 'gyp dictates that commands are run from the base directory'
(ninja.py:1613), ninja still expects the depfile path for the depfile
that this command will create to be relative to the build directory.

If the depfile argument doesn't contain special paths like
<(PRODUCT_DIR), <(SHARED_INTERMEDIATE_DIR) or <(INTERMEDIATE_DIR),
ExpandSpecial returns its argument unchanged.

If it does contain one of these special paths, on the other hand, ninja
will never find the correct depfile without this fix.

Bug: 549
Change-Id: Idfae9dd1e7134720c5ce19fc8e4f877dde796d56
Reviewed-on: https://chromium-review.googlesource.com/c/external/gyp/+/1561250


Reviewed-by: Mark Mentovai <mark@chromium.org>