[webkit-gtk] PRODUCTION_MODE is becoming DEVELOPER_MODE

[Martin Robinson]

Just a heads up to those who build the GTK+ port without using
build-webkit. In https://bugs.webkit.org/show_bug.cgi?id=132370 I
transform PRODUCTION_MODE into DEVELOPER_MODE in order make it easier
to build the GTK+ port for downstream distributions. If you are
building without build-webkit you will need to pass
-DDEVELOPER_MODE=ON when invoking CMake once this patch lands.

--Martin