cmake: Require GCC 4.9.
cmake: Require GCC 4.9.

It was been effectively required for a while, but CMake was only
warning if requirement not met.

apitrace/apitrace,schulmar/apitrace,schulmar/apitrace,trtt/apitrace,schulmar/apitrace,trtt/apitrace,apitrace/apitrace,trtt/apitrace,schulmar/apitrace,trtt/apitrace,schulmar/apitrace,apitrace/apitrace,apitrace/apitrace,trtt/apitrace