$NetBSD$

Allow setting PREFIX in Makefile.

--- Makefile.orig	2019-04-24 20:15:58.000000000 +0000
+++ Makefile
@@ -11,7 +11,7 @@ DISTFILES = src \
 	COPYING \
 	Makefile \
 
-PREFIX = /usr/local
+PREFIX?= /usr/local
 TARGET_INCLUDE_PATH = $(PREFIX)/include/avr
 
 CDEFS = -DDEFAULT_INCLUDE_PATH='"$(TARGET_INCLUDE_PATH)"' \
