Add --compiler argument to g-ir-scanner

We currently use the CC environment variable to find the C compiler to use internally in g-ir-scanner. Build systems might wish to store the compiler detected during the build configuration, and then pass that compiler to g-ir-scanner at the time of build, avoiding to put things into the environment.

One possible solution is to have a command line argument that lets us specify the C compiler, with the same semantics as the CC environment variable.