Help packages to avoid linker symbol clashes

(Imported from [Trac #701](http://hackage.haskell.org/trac/hackage/ticket/701), reported by @dcoutts on 2010-06-14)

Currently it is all too easy to end up with silently clashing linker symbols.

The prime example is C code linked into a Haskell package. Multiple versions of the same package will define the same linker symbols. The system linker is happy to just pick the first occurrence of a symbol (though the ghci linker is stricter). This can cause havoc if the behaviour of the C code changes from one version of the package to another (see for example the problem with mmap and darcs, ticket #700).

There are a number of things we could do to improve the situation.

• We could make it easier to make versioned C symbols in the C code used in a Haskell package by providing cpp macros.
  
• We could explicitly check for unversioned C symbols in Haskell libs
  
• We could try linking Haskell libs in such a way that the C symbols are not visible and thus cannot clash, but it would also mean they are not exported for other libs to use.