The setup.py file contains this:

def has_function(function_name, include_dirs=None, libraries=None, library_dirs=None):
  """Checks if a given functions exists in the current platform."""
  compiler = distutils.ccompiler.new_compiler()
  with muted(sys.stdout, sys.stderr):
      result = compiler.has_function(
          function_name,
          include_dirs=include_dirs,
          libraries=libraries,
          library_dirs=library_dirs)
  if os.path.exists('a.out'):
    os.remove('a.out')
  return result

And with the current Fedora version of python3-setuptools, this triggers an implicit function declaration for memmem:

    if has_function('memmem'):
      module.define_macros.append(('HAVE_MEMMEM', '1'))

(And for strlcpy, strlcat as well, but those are fine because we do not actually implement them.)

I hope we can patch python3-setuptools soon, so that we do not need to workaround this in python-yara.