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

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 ghc/ghc#25767 (comment 610831) for
some discussion about how this would be useful to implement
system-cxx-std-lib in userspace.

Fixes #10797