Refuse to use Anaconda's GCC

It turns out that installing GHC with ghcup results in a broken installation when Anaconda is installed. The problem is that Anaconda apparently doesn't install gmp by default and apparently Anaconda's GCC is apparently unable to find it even if it is installed.

Since Anaconda is a fairly widely-used project we should ensure that ghcup handles this somewhat gracefully.

I see two ways forward here:

• Teach ghcup to simply fail if it finds an Anaconda GCC, telling the user to specify a usable toolchain, or
• Teach ghcup to specify the explicit path to gcc (and ld, etc.) when running the GHC bindist configure script. I can see a few approaches to locate these tools:
  • specify these paths in the distribution metadata
  • do a search of PATH looking for an acceptable toolchain

There is also a related GHC bug (#18908) here since the bindist configure script really ought to fail if CC is unable to linker against gmp if the bindist was built to use GMP.