Separate build/runtime llvm60 dependencies

[Mahmoud Al-Qudsi]

Hello list,

Another complaint about `llvm60` here - I was hoping that the situation would be
ameliorated before the release of FreeBSD 12, but alas that is not the case.

>From what I understand, `mesa-dri` doesn't build out-of-the-box with `llvm70`.
Cool. But that makes `llvm60` a build dependency for `mesa-dri` - the binary
package should not require installing 813 MiB's worth of compilers,
documentation, etc. just to run something built with that toolchain.

Surely the runtime components of `llvm60` sufficient to use libraries built with
that toolchain can be split off into their own package? Even better, perhaps
`mesa-dri` can be statically compiled? (I know it has C++ components, but I'm
not sure if they ever cross the API barrier -- I haven't looked into it though,
so it's just wild speculation on my part.)

(I do see that there exists a patch for getting `mesa-dri` to build with
`llvm70` hopefully making this conversation somewhat moot, but I still strongly
object to the principle of adding such massive dependencies for common
packages.)

Mahmoud Al-Qudsi
NeoSmart Technologies