fix check_not_importing_main error on mac

Author: jepler

py/genlast.py

@@ -59,6 +59,13 @@ def maybe_preprocess(fn):
     if subprocess.call(["grep", "-lqE", "(MP_QSTR|translate)", fn]) == 0:
         preprocess(fn)
 
+# Documentation states that this has something to do with frozen programs
+# on Windows and that the line "can be omitted if the program is not going to
+# be frozen to produce an executable".  However, this exception is actually
+# occurring on Mac mpy-cross builds without this line.  Attempt to correct
+# the problem.
+multiprocessing.freeze_support()
+
 executor = ProcessPoolExecutor(max_workers=multiprocessing.cpu_count() + 1)
 executor.map(maybe_preprocess, check)
 executor.map(preprocess, always)