Added a define EXTRAMACHDEPPATH which can be used to add sys.path items
for specific platforms. Use this to add plat-mac and
plat-mac/lib-scriptpackages on MacOSX. Also tested for not having adverse
effects on Linux, and I think this code isn't used on Windows anyway.

Fixes #661521.
diff --git a/Modules/Setup.dist b/Modules/Setup.dist
index bebcd08..0c7722f 100644
--- a/Modules/Setup.dist
+++ b/Modules/Setup.dist
@@ -92,12 +92,13 @@
 
 # Path components for machine- or system-dependent modules and shared libraries
 MACHDEPPATH=:plat-$(MACHDEP)
+EXTRAMACHDEPPATH=
 
 # Path component for the Tkinter-related modules
 # The TKPATH variable is always enabled, to save you the effort.
 TKPATH=:lib-tk
 
-COREPYTHONPATH=$(DESTPATH)$(SITEPATH)$(TESTPATH)$(MACHDEPPATH)$(TKPATH)
+COREPYTHONPATH=$(DESTPATH)$(SITEPATH)$(TESTPATH)$(MACHDEPPATH)$(EXTRAMACHDEPPATH)$(TKPATH)
 PYTHONPATH=$(COREPYTHONPATH)