I might be extrapolating, but AIUI: Having two versions of LLVM in use by a binary is going to fail in surprising ways.
Base can be several years old at some point, increasing the odds of the LLVM non-compatibility biting us. And libGL is very widely used. OpenSSL is similar in this regard: replacing OpenSSL with the pkgsrc version has problems if something uses Heimdal, which dynamically links with base OpenSSL. I didn't realize we are not installing libLLVM yet. I was trying to avoid having too many outstanding changes, and needed help with the generated headers. For OpenSSL, we get by by only using base OpenSSL, but the existing status for LLVM is that pkgsrc LLVM is used a lot. So it's a ticking time bomb unless something is changed. I'm very open minded about alternative ways of getting modern Mesa. Joerg, what do you think is a working way of doing it?