[cairo] Making cairo_error weak

[mental at rydia.net]

Quoting Keith Packard <keithp at keithp.com>:

> I've been fairly happy using environment variables to control
> diagnostic printfs of this nature. It means you can see problems
> even when the application developer hasn't made any provisions for
> such.

So long as the messages are off by default.  Unexpected diagnostic
printfs can be very obnoxious on e.g. win32, as I experienced with
libart.

-mental