cmake: use environment variables

[reibax-marcus]

On cross-compile environments it is sometimes advantageous to use environment variables while building. This gives the project more flexibility and better compatibility with different build systems.

[reibax-marcus]

It works on my build environment, but I'm not 100% sure it will in it's original build environment. Please, give it a try and let me know if there's anything that needs to be changed in order to keep it working in all build environments.