set default compiler to clang in the makefile

[AndyGrant]

There are large OpenBench changes that have just been pushed, to allow for workers to be able to run just a subset of engines. IE, the worker will look for clang, gcc, g++, cargo, ... and only play workloads it can compile. As a result, I programmed a "tier list" for compilers. Demolito prefers clang. If clang is not found, and gcc is, then the make command will include CC=gcc. You may change CC=clang back to normal.

[lucasart]

Thanks. Done.