diff -u -r ../giflib-5.1.9/Makefile ./Makefile
--- ../giflib-5.1.9/Makefile	2019-03-28 18:57:23.000000000 +0000
+++ ./Makefile	2019-03-29 22:55:30.125971028 +0000
@@ -14,7 +14,7 @@
 TAR = tar
 INSTALL = install
 
-PREFIX = /usr/local
+PREFIX ?= /usr/local
 BINDIR = $(PREFIX)/bin
 INCDIR = $(PREFIX)/include
 LIBDIR = $(PREFIX)/lib
