Honor paths and compiler flags supplied by the ports framework.  Keep
upstream's warnings and avoid adding the include directories twice.

Index: mf/Makefile.OpenBSD
--- mf/Makefile.OpenBSD.orig
+++ mf/Makefile.OpenBSD
@@ -1,17 +1,19 @@
 # XFile OpenBSD Makefile
 
-PREFIX = /usr/local
-MANDIR = /usr/local/man
-SHAREDIR = $(PREFIX)/share/xfile
-APPLRESDIR = /usr/X11R6/lib/X11/app-defaults
+PREFIX ?= /usr/local
+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
+X11BASE ?= /usr/X11R6
+LOCALBASE ?= /usr/local
+INCDIRS += -I${X11BASE}/include -I${LOCALBASE}/include
+LIBDIRS += -L${X11BASE}/lib -L${LOCALBASE}/lib
 
-CFLAGS = -O2 -Wall
+CFLAGS += -Wall
 
 .c.o:
-	$(CC) $(CFLAGS) $(INCDIRS) -c -o $(@D)/$(@F) $<
+	$(CC) $(CFLAGS) -c -o $(@D)/$(@F) $<
 
 include common.mf
 
