Makefile: Improve gcc detection

The Makefile's gcc autodetection wasn't working for me, so I fixed it.
Turns out gcc is extremely abrasive towards you getting its real name.
Most of the time it gives you argv[0] (typically cc).

Meanwhile clang will happily tell me that its clang wherever possible.

Please tell me if this works on your machine.
It'll be just wonderful if gcc's behavior differs between platforms.