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I was able to resolve this. The trouble seems to be that the original scikit-build with distutils did a lot on its own to include necessary f2py libraries, and I wasn't including the right.

The call to f2py, and subsequent code adding the library and linking it, should be this:

add_custom_command(
  OUTPUT calc11module.c calc11-f2pywrappers.f calc11-f2pywrappers2.f90
  DEPENDS ${includes}
  VERBATIM
  COMMAND "${Python_EXECUTABLE}" -m numpy.f2py
          "${fortran_src_file}" -m ${f2py_module_name} --lower
  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
)

python_add_library(
        ${f2py_module_name}
       MODULE
       "${CMAKE_CURRENT_BINARY_DIR}/calc11-f2pywrappers2.f90"
       "${CMAKE_CURRENT_BINARY_DIR}/calc11-f2pywrappers.f"
       "${CMAKE_CURRENT_BINARY_DIR}/calc11module.c"
       "${fortran_src_file}"
       WITH_SOABI)
target_link_libraries(${f2py_module_name} PRIVATE fortranobject)

The "f2pywrapper" files provide the necessary API for Python functions. This links correctly, and now it runs.

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