Pass CXX and CXXFLAGS argument to ./configure scripts for build-type: Configure

[mpickering]

Since the compiler since ghc-9.4.8 requires a C++ toolchain, and that is configured into the compiler --info, it makes sense to pass that information onto the ./configure script.

The assumption that the C++ compiler is part of the toolchain is only since ghc-9.4. Therefore, if no C++ compiler is found, then the options won't be passed to the configure script.

For compilers older than 9.4, the C++ compiler will always be found as it is configured into the settings file.

See https://gitlab.haskell.org/ghc/ghc/-/issues/25767#note_610831 for some discussion about how this would be useful to implement system-cxx-std-lib in userspace.

Fixes #10797

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[Ericson2314]

Seems basically good. I might try to make more gpp g++ or gxx, but I suppose I doesn't matter to much.

I don't really get why Cabal is asking GHC what toolchains to use, but I suppose that is just inheriting the current behavior, so better to be consistent.

[mpickering]

Any reviewers? @Ericson2314 could you approve please?