Extract use_system_foo logic out of gyp files, patch 2

This removes use_system_foo logic completely from:
 * jsoncpp.gyp
 * libevent.gyp
 * libpng.gyp
 * libusb.gyp
 * libwebp.gyp
 * opus.gyp
 * re2.gyp
 * zlib.gyp

R=rsleevi
TBR=darin

BUG=226860

Review URL: https://codereview.chromium.org/14203006

git-svn-id: http://src.chromium.org/svn/trunk/src/third_party/jsoncpp@196551 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed
tree: e781275155d0d72c53e2b609271e91a6e1c1d026
  1. LICENSE
  2. README.chromium
  3. jsoncpp.gyp
  4. overrides/