Makefile: add manual path for OpenBSD

Reported by fossy <fossy@dnmx.org>, thanks

config.mk

@@ -19,6 +19,7 @@ FREETYPELIBS = -lfontconfig -lXft
 FREETYPEINC = /usr/include/freetype2
 # OpenBSD (uncomment)
 #FREETYPEINC = ${X11INC}/freetype2
+#MANPREFIX = ${PREFIX}/man
 
 # includes and libs
 INCS = -I${X11INC} -I${FREETYPEINC}