[build] Use os.sched_getaffinity only if available

This change un-breaks the build in macOS and Windows environments, by
only calling the Python os.sched_getaffinity() function where it’s
available (which seems to be only on Linux) — and where it’s not
available, using multiprocessing.cpu_count() instead.

Otherwise, the build fails in macOS and Windows environment, because the
document/core/util/mathjax2katex.py program fails — and fails the build —
with a “module 'os' has no attribute 'sched_getaffinity'” message
(because os.sched_getaffinity(0) is hardcoded into the program).

document/core/util/mathjax2katex.py

@@ -2,6 +2,7 @@
 # -*- coding: latin-1 -*-
 
 import queue
+import multiprocessing
 import os
 import re
 import shelve
@@ -229,7 +230,8 @@ def Worker():
       sys.stderr.write('.')
 
   q = queue.Queue()
-  for i in range(len(os.sched_getaffinity(0))):
+  for i in range(len(os.sched_getaffinity(0)) if "sched_getaffinity" in dir(os)
+                 else multiprocessing.cpu_count()):
     t = threading.Thread(target=Worker)
     t.daemon = True
     t.start()