Configure fails to find mfem submodule, and otherwise the build fails with the mfem-4.8 package

With the git submodule configure error is:
```
-- Could NOT find MFEM (missing: MFEM_DIR)
CMake Error at cmake/thirdparty/FindMFEM.cmake:68 (message):
Failed to find any MFEM build configuration files in
/usr/ports/science/serac/work/smith-ad49974/mfem
Call Stack (most recent call first):
cmake/thirdparty/SetupSeracThirdParty.cmake:200 (include)
CMakeLists.txt:172 (include)
```

The build with the mfem-4.8 package fails like this:
```
In file included from /usr/ports/science/serac/work/smith-ad49974/src/serac/physics/state/finite_element_vector.cpp:7:
In file included from /usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/physics/state/finite_element_vector.hpp:26:
In file included from /usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/functional.hpp:28:
In file included from /usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tensor.hpp:2112:
/usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tuple_tensor_dual_functions.hpp:742:31: error: expected ';' after expression
742 | mfem::DenseMatrixEigensystem eig_sys(matA);
| ^
| ;
/usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tuple_tensor_dual_functions.hpp:742:9: error: no member named 'DenseMatrixEigensystem' in namespace 'mfem'
742 | mfem::DenseMatrixEigensystem eig_sys(matA);
| ~~~~~~^
/usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tuple_tensor_dual_functions.hpp:742:32: error: use of undeclared identifier 'eig_sys'
742 | mfem::DenseMatrixEigensystem eig_sys(matA);
| ^
/usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tuple_tensor_dual_functions.hpp:743:3: error: use of undeclared identifier 'eig_sys'
743 | eig_sys.Eval();
| ^
/usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tuple_tensor_dual_functions.hpp:749:17: error: use of undeclared identifier 'eig_sys'
749 | output[k] = eig_sys.Eigenvalue(k);
| ^
/usr/ports/science/serac/work/smith-ad49974/src/serac/infrastructure/../../serac/numerics/functional/tuple_tensor_dual_functions.hpp:753:72: error: use of undeclared identifier 'eig_sys'
753 | tensor phi = make_tensor([&](int i) { return eig_sys.Eigenvector(k)[i]; });
| ^
6 errors generated.
```

rev.ad49974
OS: FreeBSD 15 STABLE
clang-19

Reproduce on FreeBSD 15 STABLE with clang-19 using both the MFEM submodule and the mfem-4.8 package. Inspect cmake/thirdparty/FindMFEM.cmake, cmake/thirdparty/SetupSeracThirdParty.cmake, CMakeLists.txt, and tuple_tensor_dual_functions.hpp around line 742; done means configuration finds MFEM and the package build no longer reports the shown missing symbol.

Written by the indexing model from the issue text.