Make generator: Remove duplicate library entries.
Make generator: Remove duplicate library entries.

BUG=none
TEST=No more -lpthread -lpthread ... -lpthread on the command line.
Review URL: http://codereview.chromium.org/582025
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google