ghc-toolchain: Environment variables should be used to specify toolchain

At the moment ghc-toolchain is still very reliant on ./configure because it doesn't look at any environment variables.

It's pretty common in packaging situations to specify CC/CXX/LD etc, so I think that ghc-toolchain should also look at these variables in order to match what ./configure does.

This will also resolve some awkward interaction between ./configure and ghc-toolchain where we have to save a bunch of options into USER_* variants because we can run ghc-toolchain much earlier (just after the windows toolchain is setup) before ./configure has modified the values of these variables.