Use ports infrastructure variables for paths.

--- mf/Makefile.OpenBSD.orig
+++ mf/Makefile.OpenBSD
@@ -1,10 +1,10 @@
 # XFile OpenBSD Makefile
 
-PREFIX = /usr/local
-MANDIR = /usr/local/man
-SHAREDIR = $(PREFIX)/share/xfile
-APPLRESDIR = /usr/X11R6/lib/X11/app-defaults
+PREFIX ?= ${PREFIX}
+MANDIR ?= ${PREFIX}/man
+SHAREDIR ?= $(PREFIX)/share/xfile
+APPLRESDIR ?= ${PREFIX}/lib/X11/app-defaults
 
-INCDIRS = -I/usr/X11R6/include -I/usr/local/include
-LIBDIRS = -L/usr/X11R6/lib -L/usr/local/lib
+INCDIRS += -I${X11BASE}/include -I${LOCALBASE}/include
+LIBDIRS += -L${X11BASE}/lib -L${LOCALBASE}/lib
 
 CFLAGS = -O2 -Wall
