Building against external LLVM

[barracuda156]

@thepowersgang Could we add a non-default option to build against existing LLVM installation? That should be supported in principle:
https://internals.rust-lang.org/t/building-with-system-llvm/1647
https://internals.rust-lang.org/t/solved-avoid-building-llvm/10243

This is desirable for Darwin on powerpc, since upstream LLVM is broken, and we will need to build with patches from @iains to make it usable, but we already have it built that way (I do, anyway), and it is a waste to increase compilation time by several hours.

[thepowersgang]

I'm open to a merge request with that option added to the makefiles. It's not something my setup is configured to use, so not immediately possible (and I'm in the middle of the 1.90 upgrade)

[barracuda156]

I will see if it can be easily done in a non-intrusive way. I have switched locally to my own LLVM, and I think it works (I still have unsolved issues, but they seem to be unrelated).