Process all input files in parallel
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
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google