Process all input files in parallel

When given several input files on the command line in parallel mode,
load them all in parallel instead of doing a bunch of separate parallel
runs.

Fixes gyp --parallel for projects with multiple .gyp files.

Patch from simonb@opera.com

BUG=gyp:347
R=dmazzoni@chromium.org, scottmg@chromium.org

Review URL: https://codereview.chromium.org/23475007

git-svn-id: http://gyp.googlecode.com/svn/trunk@1753 78cadc50-ecff-11dd-a971-7dbc132099af