Add option to disable generating embedded LLVM bitcode

From build errors reported on the mailing list, it appears that setting up LLVM/GCC correctly is not so easy on older systems when you have different GCC versions installed. In that case clang++ can fail to compile the bitcode.

I don't know if this option is strictly necessary, maybe solving this is just a matter of building LLVM correctly, but it was already disabled on Windows so might as well make it an option.