re: lib/60324: unwind.h build race condition

sorry, i'd missed this PR being submitted.

i think the fix you commited is fine, and nicely found on what
is likely going wrong (i'm convinced by your analysis.)


maybe the real fix is to not enter this subdirectory twice in
this same high-level .WAIT group at all.

there are a few subdirs we traverse twice but perhaps we should
stop doing that entirely.  eg, src/Makefile had (the first line
would have a .WAIT now):

_SUBDIR=        tools .WAIT lib
_SUBDIR+=       include external crypto/external bin

and src/external/Makefile has:

SUBDIR+= amdgpu-firmware apache2 atheros broadcom bsd cddl gpl2 gpl3 historical

and src/external/gpl3/Makefile has:

SUBDIR+=        ${EXTERNAL_GCC_SUBDIR}

etc., where it ends up entering libgcc from multiple parts of
the same .WAIT segment.

this isn't the only one, but maybe it's the only one that has
a build-a-file conflict.  not doing this would mean we could
take the new .WAIT away again.

it might be nice to still allow this with some variable set,
since eg, i will cleandir in src/external/gpl3/gcc and rebuild
just that sometimes when i'm testing gcc stuff, but i'm fine
for this to be a special thing you have to ask for.)


thanks.


.mrg.