Avoid the use of 'unset', which isn't available on all platforms. Fixes SF
bug #442627.
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 7fd60a1..d073c01 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -286,7 +286,7 @@
 
 # Build the shared modules
 sharedmods: $(PYTHON)
-	unset PYTHONPATH PYTHONHOME PYTHONSTARTUP; \
+	PYTHONPATH= PYTHONHOME= PYTHONSTARTUP= \
 		./$(PYTHON) $(srcdir)/setup.py build
 
 # buildno should really depend on something like LIBRARY_SRC