Enable AUTO_OBJ by default if the OBJDIR is writable, only for in-tree builds.

Enable AUTO_OBJ by default if the OBJDIR is writable, only for in-tree builds.

This can be disabled by putting WITHOUT_AUTO_OBJ=yes in /etc/src-env.conf, not
/etc/src.conf, or passing it in the environment.

The purpose of this rather than simply flipping the default of AUTO_OBJ to yes
is to avoid hassling users with auto.obj.mk failures if the wanted OBJDIR is
not writable. It will fallback to writing to the source directory like it does
today if MAKEOBJDIRPREFIX is not writable.

The act of enabling MK_AUTO_OBJ disables all 'make obj' treewalks since
previous work has made those not run if MK_AUTO_OBJ==yes in Makefile.inc1.

Relnotes:	yes
Reviewed by:	sjg
Discussed at:	https://lists.freebsd.org/pipermail/freebsd-arch/2016-May/017805.html
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D12841