Makefile: Fix MAKEOBJDIRPREFIX command-line variable check for bmake

Unlike the old fmake, running make FOO=bar when using bmake doesn't put
FOO=bar in .MAKEFLAGS at the top level, it instead just puts FOO in
.MAKEOVERRIDES and the full MAKEFLAGS will be formed for sub-makes.
Moreover, this only applies for sub-makes in rules, so this doesn't
apply to those in shell assignments. This means that the current check
does not catch make MAKEOBJDIRPREFIX=..., only those defined in config
files. Thus we must also check .MAKEOVERRIDES explicitly.

Reviewed by:	sjg
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D31015

Makefile

@@ -218,7 +218,7 @@ _MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} MK_AUTO_OBJ=no \
     ${.MAKEFLAGS:MMAKEOBJDIRPREFIX=*} __MAKE_CONF=${__MAKE_CONF} \
     SRCCONF=${SRCCONF} SRC_ENV_CONF= \
     -f /dev/null -V MAKEOBJDIRPREFIX dummy
-.if !empty(_MAKEOBJDIRPREFIX)
+.if !empty(_MAKEOBJDIRPREFIX) || !empty(.MAKEOVERRIDES:MMAKEOBJDIRPREFIX)
 .error MAKEOBJDIRPREFIX can only be set in environment or src-env.conf(5),\
     not as a global (in make.conf(5) or src.conf(5)) or command-line variable.
 .endif