Build a static library on macOS

[connorjward]

Over the past 1/2 weeks something has gone buggy in the macOS toolchain (current suspect is gfortran 16.2 but that is a total shot in the dark).

In Firedrake this has manifested as a missing symbol error when the smoke tests are run on macOS:

 ==================================== ERRORS ====================================
  _ ERROR collecting tests/firedrake/supermesh/test_assemble_mixed_mass_matrix.py _
  ImportError while importing test module '/Users/github/actions-runner/_work/firedrake/firedrake/venv/lib/python3.14/site-packages/firedrake/_check/tests/firedrake/supermesh/test_assemble_mixed_mass_matrix.py'.
  Hint: make sure your test modules/packages have valid Python names.
  Traceback:
  /opt/homebrew/Cellar/python@3.14/3.14.7/Frameworks/Python.framework/Versions/3.14/lib/python3.14/importlib/__init__.py:88: in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  tests/firedrake/supermesh/test_assemble_mixed_mass_matrix.py:3: in <module>
      from firedrake.supermeshing import *
  ../supermeshing.py:8: in <module>
      from firedrake.cython.supermeshimpl import assemble_mixed_mass_matrix as ammm, intersection_finder
  E   ImportError: dlopen(/Users/github/actions-runner/_work/firedrake/firedrake/venv/lib/python3.14/site-packages/firedrake/cython/supermeshimpl.cpython-314-darwin.so, 0x0002): symbol not found in flat namespace '_libsupermesh_tree_intersection_finder_set_input'

I have tracked this missing symbol error down to the following:

1. The symbol comes from https://github.com/firedrakeproject/libsupermesh/blob/main/src/C_bindings.F90
2. If I compile that file into an object file and run

$ nm -gU CMakeFiles/supermesh.dir/src/C_bindings.F90.o | grep tree_intersection_finder_set_input

I see

0000000000000488 T _libsupermesh_tree_intersection_finder_set_input

I.e. the symbol exists in the object file

3. If I then link this file into the full executable and run

nm -gU libsupermesh.dylib | grep tree_intersection_finder_set_input

I see

000000000000b490 T ___libsupermesh_intersection_finder_MOD_tree_intersection_finder_set_input

the symbol is missing.

The 'fix' I have found is that the symbol appears to be correctly exported in a static library, as opposed to a shared one. I have tested Firedrake and this fixes the issue.