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/Makefile.pre.in b/Makefile.pre.in
index c767dd5..333f348 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -612,6 +612,7 @@
 # Install the library
 PLATDIR=	plat-$(MACHDEP)
 EXTRAPLATDIR= @EXTRAPLATDIR@
+EXTRAMACHDEPPATH=@EXTRAMACHDEPPATH@
 MACHDEPS=	$(PLATDIR) $(EXTRAPLATDIR)
 XMLLIBSUBDIRS=  xml xml/dom xml/parsers xml/sax
 PLATMACDIRS= plat-mac plat-mac/Carbon plat-mac/lib-scriptpackages \
@@ -622,6 +623,7 @@
 	plat-mac/lib-scriptpackages/Netscape \
 	plat-mac/lib-scriptpackages/StdSuites \
 	plat-mac/lib-scriptpackages/Terminal
+PLATMACPATH=:plat-mac:plat-mac/lib-scriptpackages
 LIBSUBDIRS=	lib-old lib-tk site-packages test test/output test/data \
 		encodings email email/test email/test/data compiler hotshot \
 		logging bsddb bsddb/test \