cmake: Require GCC 4.6.
cmake: Require GCC 4.6.

Although earlier GCC versions have some C++11 support, 4.6 is the first
to support things like null pointer constant.  Above all, GCC 4.6 is the
oldest version I plan to regularly build and support.

If people need to build apitrace with older GCC version, then the best
thing is to build apitrace release 6.1.

See issue #328.

trtt/apitrace,EoD/apitrace,EoD/apitrace,trtt/apitrace,trtt/apitrace,EoD/apitrace,joshua5201/apitrace,surround-io/apitrace,joshua5201/apitrace,tuanthng/apitrace,schulmar/apitrace,trtt/apitrace,joshua5201/apitrace,swq0553/apitrace,joshua5201/apitrace,tuanthng/apitrace,swq0553/apitrace,schulmar/apitrace,schulmar/apitrace,apitrace/apitrace,swq0553/apitrace,surround-io/apitrace,tuanthng/apitrace,EoD/apitrace,EoD/apitrace,apitrace/apitrace,tuanthng/apitrace,surround-io/apitrace,swq0553/apitrace,apitrace/apitrace,surround-io/apitrace,swq0553/apitrace,joshua5201/apitrace,schulmar/apitrace,apitrace/apitrace,trtt/apitrace,tuanthng/apitrace,surround-io/apitrace,schulmar/apitrace