Use PETSc CXX for C++ extension builds

[gthyagi]

Summary

• read CXX from PETSc petscvariables in setup.py
• export CXX alongside the existing CC override

Why this is important

Pre-fix, setup.py only forced PETSc's CC. That meant C builds followed PETSc, but C++ builds could still fall back to the ambient environment compiler.

In practice, this can send underworld3.ckdtree through a different or broken C++ toolchain than the PETSc/MPI build, especially in Pixi/OpenMPI environments.

Using both PETSc CC and CXX keeps the Underworld extension build aligned with the PETSc toolchain.

Validation

• python3 -m py_compile setup.py
• OMPI_CC=/usr/bin/clang OMPI_CXX=/usr/bin/clang++ pixi run -e amr-dev pip install . --no-build-isolation

[gthyagi]

Added a follow-up fix for runtime JIT builds. The earlier change made package builds use PETSc's , but was still spawning temporary builds without that compiler environment. This commit now passes PETSc-derived into the JIT subprocess and sets when the OpenMPI wrapper points to a missing compiler name. Verified by rerunning the three previously failing collection-time tests: , , and ().

[gthyagi]

Added a follow-up fix for runtime JIT builds.

The earlier setup.py change made package builds use PETSc CC/CXX, but _jitextension.py was still spawning temporary setup.py build_ext --inplace builds without that compiler environment. This commit now passes PETSc-derived CC/CXX into the JIT subprocess and sets OMPI_CC/OMPI_CXX when the OpenMPI wrapper points to a missing compiler name.

Verified by rerunning the three previously failing collection-time tests:

• tests/test_0050_utils.py
• tests/test_1004_DarcyCartesian.py
• tests/test_1011_stokesSph.py

Result: 6 passed.