$NetBSD$

Allow setting PREFIX.

--- Makefile.orig	2019-02-19 18:27:53.000000000 +0000
+++ Makefile
@@ -22,7 +22,7 @@ CFLAGS ?= -g -O2
 CFLAGS += -Wall -Wextra -Wconversion
 CFLAGS += -D_FILE_OFFSET_BITS=64
 
-PREFIX = /usr/local
+PREFIX?= /usr/local
 DESTDIR =
 
 bindir = $(PREFIX)/bin