This is now controlled at the top level goji package: these packages are
not even compiled in when using App Engine. This is sensible, since
neither package is of any use there.

I originally exposed Compile() for exactly this use case, but apparently
I never actually implemented this. Oops.

In any event, this makes the first request a little faster (an extremely
unscientific test suggests on the order of 10 microseconds). Also, if
something goes terribly wrong during route compilation, it would fail
before we start listening on the socket.