Deal with src.conf for top-level MAKEOBJDIRPREFIX guard.

- Don't discard SRCCONF value since it may incorrectly have MAKEOBJDIRPREFIX
  in it.
- Add note about src.conf not being a suitable place for MAKEOBJDIRPREFIX.

Sponsored by:	Dell EMC Isilon

Makefile

@@ -191,10 +191,11 @@ PATH=	/sbin:/bin:/usr/sbin:/usr/bin
 MAKEOBJDIRPREFIX?=	/usr/obj
 _MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} MK_AUTO_OBJ=no \
     ${.MAKEFLAGS:MMAKEOBJDIRPREFIX=*} __MAKE_CONF=${__MAKE_CONF} \
+    SRCCONF=${SRCCONF} \
     -f /dev/null -V MAKEOBJDIRPREFIX dummy
 .if !empty(_MAKEOBJDIRPREFIX)
 .error MAKEOBJDIRPREFIX can only be set in environment, not as a global\
-	(in make.conf(5)) or command-line variable.
+	(in make.conf(5) or src.conf(5)) or command-line variable.
 .endif
 
 # We often need to use the tree's version of make to build it.