Honor paths and flags supplied by the ports framework.  Correct the
OpenMP option name and keep OpenMP disabled unless explicitly enabled.

Index: mf/Makefile.OpenBSD
--- mf/Makefile.OpenBSD.orig
+++ mf/Makefile.OpenBSD
@@ -5,18 +5,20 @@
 # -----------------------------------------------------------------------------
 # XImaging OpenBSD Makefile
 
-ENABLE_OPM = 0
-ENABLE_CDE = 0
+ENABLE_OMP ?= 0
+ENABLE_CDE ?= 0
 
-PREFIX = /usr/local
-MANDIR = /usr/local/man
-APPLRESDIR = /usr/X11R6/lib/X11/app-defaults
+PREFIX ?= /usr/local
+MANDIR ?= ${PREFIX}/man
+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 $(INCDIRS)
-LDFLAGS = -pthread
+CFLAGS += -Wall
+LDFLAGS += -pthread
 
 .if $(ENABLE_OMP) == 1
 CFLAGS += -fopenmp -DENABLE_OMP
