dirdeps.mk: move logic to handle -f dirdeps.mk to inside check
for first read of dirdeps.mk
Also fix handling of WITHOUT_DIRDEPS_BELOW
gendirdeps.mk: pass M2D_EXCLUDES to meta2deps
meta.autodep.mk: if we build with nofilemon, leave a cookie to
prevent updating dependencies until cleaned.
Reviewed by: bdrewery
Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.
Still need to add real targets under targets/ to build packages.
Differential Revision: D2796
Reviewed by: brooks imp
find oursleves creating a machine qualified .MAKE.DEPENDFILE which
is identical to a default which exists - skip it.
This allows us to create Makefile.depend.host where it matters,
while leveraging Makefile.depend where it is sufficient.
