Automatically set --pgmcxx for C++ sources when a C++ compiler is available

**Describe the bug** Cabal does not set `-pgmcxx` for GHC when compiling C++ sources, even when a separate C++ compiler (`g++`) is available. By default, GHC falls back to the same compiler used for C, which does **not** automatically link the C++ standard library. This forces users to add `extra-libraries: stdc++` to every project using `cxx-sources`. This is particularly problematic for GHC 9.4+ and for users who install GHC via ghcup, as ghcup always provides both `gcc` and `g++`. Moreover, Cabal's own `configureToolchain` and `runConfigureScript` logic already uses `g++` in some places, leading to an inconsistency where C++ sources compiled through GHC are treated differently (with `gcc`) than other C++ compilation steps. **To Reproduce** 1. Create a minimal project with a C++ source file, intentionally **without** `-pgmcxx` or `extra-libraries`: ```cabal executable test-cxx main-is: Main.hs build-depends: base default-language: Haskell2010 include-dirs: cxxbits cxx-sources: cxxbits/lib.cpp ``` 2. Try to build: ``` $ cabal build ``` Observe the linking error due to missing `stdc++`. **Expected behavior** If `g++` is present, Cabal should automatically pass `-pgmcxx=g++` to GHC, so that C++ sources are compiled and linked correctly without manual `extra-libraries`. The project above should build successfully out-of-the-box, and the behaviour should be consistent with how `configureToolchain` already prefers `g++` for C++-related tasks. **Additional context** - Addressed by PR #11763 - The internal `configureToolchain` and `runConfigureScript` already use `g++`, creating inconsistency with C++ source compilation through GHC.