FAST_DEPEND: Allow not using -MP by setting DEPEND_MP blank.

-MP creates empty targets for all dependency files, which can be useful when a
dependency is deleted from the file system.  This would otherwise cause an
error for "don't know how to build FOO" since the .depend file is included
with the dependency registered.

This is mostly a workaround for the misc/dahdi-kmod port using '::' for one of
its dependencies, while -MP uses just ':'.  This results in an 'Inconsistent
operator for' error.

X-MFC-With:	r290433
MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division

share/mk/bsd.dep.mk

@@ -59,7 +59,8 @@ DEPENDFILE?=	.depend
 DEPENDFILES=	${DEPENDFILE}
 .if ${MK_FAST_DEPEND} == "yes" && ${.MAKE.MODE:Unormal:Mmeta*} == ""
 DEPENDFILES+=	${DEPENDFILE}.*
-DEPEND_CFLAGS+=	-MD -MP -MF${DEPENDFILE}.${.TARGET}
+DEPEND_MP?=	-MP
+DEPEND_CFLAGS+=	-MD -MF${DEPENDFILE}.${.TARGET}
 DEPEND_CFLAGS+=	-MT${.TARGET}
 CFLAGS+=	${DEPEND_CFLAGS}
 DEPENDOBJS+=	${OBJS} ${POBJS} ${SOBJS}