Deal with MAKEOBJDIRPREFIX

MAKEOBJDIRPREFIX does not really fit our model.
Use it to set OBJROOT to about where user expects
and set MAKEOBJDIR to do what we want.

share/mk/local.meta.sys.mk

@@ -8,6 +8,18 @@
 MK_INSTALL_AS_USER= yes
 
 .if empty(OBJROOT) || ${.MAKE.LEVEL} == 0
+.if defined(MAKEOBJDIRPREFIX) && exists(${MAKEOBJDIRPREFIX})
+.warning MAKEOBJDIRPREFIX not supported; setting MAKEOBJDIR...
+# put things approximately where they want
+OBJROOT:=${MAKEOBJDIRPREFIX}${SRCTOP:S,/src,,}/
+# OBJTOP set below
+MAKEOBJDIR=$${.CURDIR:S,$${SRCTOP},$${OBJTOP},}
+MAKEOBJDIRPREFIX=
+# export but do not track
+.export-env MAKEOBJDIRPREFIX MAKEOBJDIR
+# now for our own use
+MAKEOBJDIR= ${.CURDIR:S,${SRCTOP},${OBJTOP},}
+.endif
 .if !empty(SB)
 SB_OBJROOT ?= ${SB}/obj/
 # this is what we use below