Only descend into directories if USE_NEW_TOOLCHAIN is set (allows top level
Makefile to use "-m ${.CURDIR}/share/mk" to get the "correct" <bsd.own.mk>
when descending into src/tools/Makefile).
in the rest of the source tree, for reasons that will be documented separately.
Short story: host tools that are out of date need to be rebuilt
*from scratch* to avoid our common `source skew accidents'.