Add framework to provide ad-hoc diagnostics for common failures

[duncan]

Just as we have a system for doing various QA checks on cabal packages, perhaps we should have something to help users diagnose common problems with the system environment that crop up as e.g. linker errors.

For example, a user reported in #haskell that the wx package failed to configure and reported that the stdc++ C library was missing. We eventually discovered that although the standard C++ library was installed, it was not found by gcc -lstdc++. Installing the C++ components of gcc (ie the distro package that provides g++) fixed the issue, presumably because installing that package modified the standard gcc linker search path to include the location where stdc++ was already installed.

It would be good if after diagnosing this particular issue, it was easy to code up a test for this situation and add it into Cabal's Simple build system so that other users might get more helpful diagnostics in future. It does not need to be comprehensive, so it is ok to try and grok the output of linkers and to work just on some OSs or just with gcc for example.

This might also help with common ghc problems.