Fix setup_multiprocessing.py to work on Macs.

Author: hpparvi

doc/source/examples/setup_multiprocessing.py

@@ -5,6 +5,13 @@
 os.environ["VECLIB_MAXIMUM_THREADS"] = "1"
 os.environ["NUMEXPR_NUM_THREADS"] =    "1"
 
+# I'm forcing the multiprocessing start method to 'fork' on Macs
+# because 'spawn' seems to cause errors with the numba code.
+from sys import platform
+if 'darwin' in platform:
+    import multiprocessing as mp
+    mp.set_start_method('fork')
+
 from numba import set_num_threads, config
-config.THREADING_LAYER = 'forksafe'
-set_num_threads(1)
+config.THREADING_LAYER = 'safe'
+set_num_threads(1)