Interesting also to note that any subsequent run of ./build.sh since
the first run of "distribution" causes the DESTDIR to use the latter
setting, suggesting a stray/bogus "test -d" somewhere, but I can't
find it.

  $ ./build.sh distribution
  [...]
  ===> MACHINE:          i386
  ===> MACHINE_ARCH:     i386
  ===> TOOLDIR path:     /var/build/obj/tooldir.NetBSD-2.99.10-i386
  ===> DESTDIR path:     /usr/sketch/cvs/netbsd/src/obj/destdir.i386
  ===> RELEASEDIR path:  /var/build/obj/releasedir

If I then delete the ./obj symlink and try again:

  $ rm -f obj
  $ ./build.sh distribution
  [...]
  ===> MACHINE:          i386
  ===> MACHINE_ARCH:     i386
  ===> TOOLDIR path:     /var/build/obj/tooldir.NetBSD-2.99.10-i386
  ===> DESTDIR path:     /usr/sketch/cvs/netbsd/src/destdir.i386
  ===> RELEASEDIR path:  /var/build/obj/releasedir

_SRC_TOP_OBJ_ is clearly being set differently depending on whether
the obj symlink exists or not (verified with some debugging added to
build.sh).