Print instructive error message if builtin_llvm=off

[wiene]

Fixes bug ROOT-9186.

[phsft-bot]

Can one of the admins verify this patch?

[Axel-Naumann]

Thanks, @wiene ! Indeed - we have upstreamed llvm patches but not yet all of those on clang.

@phsft-bot build!

[Axel-Naumann]

@phsft-bot build!

[phsft-bot]

Starting build on centos7/gcc49, mac1012/native, slc6/gcc49, slc6/gcc62, slc6/gcc62, ubuntu14/native, ubuntu14/native, windows10/vc15 with flags -Dvc=OFF -Dimt=ON -Dccache=ON
How to customize builds

[phsft-bot]

Build failed on centos7/gcc49.
See console output.

[wiene]

@Axel-Naumann, looking at the build logs, it seems to me that there is a problem with the CI system: "No space left on device". Could you have a look at it?

[Axel-Naumann]

Yes we're aware and working on a fix.

[vgvassilev]

LLVM is technically patch free. We can teach our build system to try to find LLVM 5.0 and use it. The tricky part is that we still need to build clang (we should probably set the LLVM_EXTERNAL_CLANG_SOURCE_DIR).

[Axel-Naumann]

Indeed, but that should happen through a different PR; the current state is best described and fixed by this PR.