Fix error message when using exclusion

Patch by Sylvain Defresne <sdefresne@chromium.org>

Review URL: https://codereview.chromium.org/64633003/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1786 78cadc50-ecff-11dd-a971-7dbc132099af
Use exec and proper quoting in top-level gyp command.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/74243002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1787 78cadc50-ecff-11dd-a971-7dbc132099af