ninja: If -Gconfig is given, write just that single config. Else, write all configs.
ninja: If -Gconfig is given, write just that single config. Else, write all configs.

jamesr used to use different GYP_DEFINES for different configs, which was broken by
making ninja always write all configs. Writing all configs unless one is passed
explicitly supports his use case while still keeping the common case (same GYP_DEFINES
for Debug and Release) easy to use.
Review URL: https://chromiumcodereview.appspot.com/9246007
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp