ghc: allow building with clang-3.4 or later

[mojca (Mojca Miklavec)]

I tried building ghc with clang 3.4 on 10.7 which worked fine. I have 3.4 and the latest one installed and it felt like a nuisance to install also version 3.5. It would be great if one could build with either clang compiler that's new enough, not just 3.5.

[ryandesign (Ryan Carsten Schmidt)]

You are referring to the llvm-3.5 library dependency? You are suggesting that the port should offer variants to let the user choose which version of llvm they would like to use?

[mojca (Mojca Miklavec)]

Yes, I'm referring to:

port:llvm-3.5

and

set llvmPrefix  ${prefix}/libexec/llvm-3.5

in the ghc port.

If this is also a runtime dependency then variants are probably the only way to implement this properly. If it is only a build-time dependency, it would be slightly easier. We do something similar to root6 for example.

If variants are not desired, it would at least make sense to pick either 3.4 (probably the lowest supported one; I didn't try 3.3, so I cannot tell) or the one that is considered "default" (reasonably high one if not the latest one). 3.5 is just weird enough that it causes problems on older OSes while not bringing much benefit over 3.4 (3.4 is often the bare minimum for C++11, so it is often installed).

[neverpanic (Clemens Lang)]

I'm planning to switch this to llvm-3.7 for the next GHC upgrade.

[mf2k (Frank Schima)]

Assigning to current maintainer. See #51780.

[mf2k (Frank Schima)]

Calling this a duplicate of #51439 or #48899.