Add support for custom install prefix

Author: Clcanny

lib/Makefile

@@ -15,10 +15,10 @@ C=${TOPDIR}/freebsd/contrib/ck/include
 MACHINE_INCLUDES_ROOT:=${CURDIR}/machine_include
 OVERRIDE_INCLUDES_ROOT:=${CURDIR}/include
 X86_INCLUDES=0
-PREFIX=/usr/local
+PREFIX?=/usr/local
 PREFIX_LIB=${PREFIX}/lib
-PREFIX_INCLUDE=/usr/local/include
-PREFIX_BIN=/usr/local/bin
+PREFIX_INCLUDE=${PREFIX}/include
+PREFIX_BIN=${PREFIX}/bin
 F-STACK_CONF=/etc/f-stack.conf
 F-STACK_VERSION=1.24
 TGT_OS=$(shell uname)