cmake: add option to use llvm-config to find cross-target llvm deps

[jjsullivan5196]

Closes #6386

[daurnimator]

With this PR ZIG_PREFER_CLANG_CPP_DYLIB will override ZIG_FORCE_LLVM_CONFIG. Maybe worth s/FORCE/PREFER ?
Or can we find dynamic libraries with llvm-config?

[jjsullivan5196]

@daurnimator Yeah, PREFER makes sense then, I'll change that out.

Also, are you suggesting there's a better way to go about solving the issue? I can try to make a more declarative option, but I didn't want to break any existing users of the cmake setup.

[daurnimator]

Also, are you suggesting there's a better way to go about solving the issue? I can try to make a more declarative option, but I didn't want to break any existing users of the cmake setup.

e.g. you might want to have this new option take precedence over ZIG_PREFER_CLANG_CPP_DYLIB

or I was wondering if llvm-config has some option that can replace ZIG_PREFER_CLANG_CPP_DYLIB's find_library in certain setups? (I vaguely recall we tried this once but it didn't work? maybe LLVM have fixed it now? or at least in the LLVM 11 release?)

[jjsullivan5196]

Also, are you suggesting there's a better way to go about solving the issue? I can try to make a more declarative option, but I didn't want to break any existing users of the cmake setup.

e.g. you might want to have this new option take precedence over ZIG_PREFER_CLANG_CPP_DYLIB

or I was wondering if llvm-config has some option that can replace ZIG_PREFER_CLANG_CPP_DYLIB's find_library in certain setups? (I vaguely recall we tried this once but it didn't work? maybe LLVM have fixed it now? or at least in the LLVM 11 release?)

I think the current precedence is fine, since ZIG_PREFER_CLANG_CPP_DYLIB is meant to override using llvm-config in the first place. I'll clarify in the doc comment for this option.

As for llvm-config replacing ZIG_PREFER_CLANG_CPP_DYLIB, I'm not sure of that myself.

[jjsullivan5196]

I changed the wording per @daurnimator's suggestion