Re-gen.

Add a -B option to set the BUILDID for the build. This also causes
the make(1) wrapper script to be named "nbmake-MACHINE-BUILDID".

document various supported environment variables, including MAKEOBJDIRPREFIX

document:
- make's UNPRIVED support
- build.sh's -U support
- the build system currently needs to build onto a file system with
long file name and hard link support

Document the -d option to build.sh

Note only <bsd.own.mk> as the authority for determining whether the new
BUILDING instructions should be used.

Turn off host-specific objdirs by default. Set MKHOSTOBJ=yes to enable.

Major overhaul of src/tools and host toolchain. Summary of changes:

* Rewrite src/tools Make logic to work like the rest of the tree wrt
"dependall" and "install". The old "make build" hack is g

Major overhaul of src/tools and host toolchain. Summary of changes:

* Rewrite src/tools Make logic to work like the rest of the tree wrt
"dependall" and "install". The old "make build" hack is gone.

* Remove the MKTOOLS logic. This was linked to the "make build" hack,
and was only needed because TOOLDIR originally had no writable default.

* Redo the GNU configure/make logic to make it fit reasonably in a
BSD make wrapper. Use new ${.ALLTARGETS} variable to scan for
targets in $(srcdir), and mark them with .MADE: to prevent rebuilding.

* Only build cross tools in src/tools; remove some messy logic in
src/usr.* and src/gnu/usr.* that would do target filename rewriting
(improves consistency and readability).

* Add the ability to build cross gdb at tool build time by setting
MKCROSSGDB (default no) to "yes" in mk.conf.

* Add src/tools/groff and set up paths to work with this cross groff.

show more ...

Remove mention of BUILDLING.html, and follow rename of BUILDING.txt to
BUILDING.
Noted by Love <lha@stacken.kth.se> on tech-toolchain.

By popular demand, one preformatted version of BUILDING, plaintext, no CRs.