Do not force the compiler version about c-ray

[vkoskiv]

Actually that set(CMAKE_C_COMPILER "gcc-9") is in there by accident. I just added it for testing, and indeed one should just do cmake . -DCMAKE_C_COMPILER=<compiler> instead. The helper scripts in the rebuild directory do just this.

And yeah, after seeing the massive performance boost from a little tweak to CMakeLists.txt, I was a bit anxious to commit it in as quickly as possible. I'll apply that check you outlined shortly.

I think requiring CMake 3.9 and newer is fine, all my experiments with running this on ancient hardware use plain shell scripts for the build system anyway.

[vkoskiv]

Resolved by d56da63