The 'modetest' test application that is compiled (but not installed) with libdrm includes optional cairo support, starting with version 2.4.5. I'd imagine that this could be enabled/disabled with a USE flag, rather than letting the configure script detect whether cairo is present - which is what seems to happen now.
Not really a bug. The "modetest" utility is not installed so it makes no sense for us to check for cairo or to add a USE flag. And even if upstream offered the option in ./configure, I think I would hard-disable it: the potential for cyclic dependencies is too important.
Thanks anyway :)