[rustbuild] should not assume llvm-config shares root with FileCheck

[nagisa]

I have a wrapper for llvm-config in ~/local/bin/ to workaround for a bug in original llvm-config, but rustbuild assumes there’s FileCheck in the same directory as llvm-config, making rustbuild fail.

It should instead get the LLVM bindir from llvm-config itself.

$ which llvm-config
/home/nagisa/local/bin/llvm-config
$ llvm-config --bindir
/usr/bin

[durka]

We also just added a config.toml option to stop it from looking for FileCheck at all.

[sanxiyn]

Another way to trigger this is llvm-config = "/usr/bin/llvm-config-3.9". bindir is /usr/lib/llvm-3.9/bin, /usr/bin/FileCheck-3.9 is a symbolic link to /usr/lib/llvm-3.9/bin/FileCheck. rustbuild tries to find /usr/bin/FileCheck which does not exist and fail.