Makefile: added instructions on how to override PREFIX

Makefile: added instructions on how to override PREFIX

--- a/Makefile	Wed Dec 23 20:34:09 2009 +0100
+++ b/Makefile	Wed Dec 23 23:28:53 2009 +0100
@@ -1,3 +1,9 @@
+# If you want to change PREFIX, do not just edit it below. The changed
+# value wont get passed on to recursive make calls. You should instead
+# override the variable on the command like:
+#
+# % make PREFIX=/opt/ install
+
 PREFIX=/usr/local
 export PREFIX
 PYTHON=python