Linux: environment variables should only impact target toolset.

The change in r1691 caused us to add environment variables to
host and target toolsets. We should only add them to target
toolsets.

Note that gyp-ninja resolves environment variables at gyp time.
However, gyp-make resolves environemnt variables at make time. 
Evan tells me that ninja needs to resolve variables at gyp time
otherwise it won't know to rebuild based on environment variable
changes.

Also fix a bunch of bugs in the test.

BUG=None
R=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/22861004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1694 78cadc50-ecff-11dd-a971-7dbc132099af