Add a double check that builddir is existing

[olivier]

Summary

I have META_MODE enabled and I meet sometime this bug:

$ make clean-jobs buildworld-jobs buildkernel-jobs
(etc.)
$ tools/build/beinstall -j1
(etc.)
===> share/zoneinfo (install)
mkdir -p /usr/share/zoneinfo
(cd /usr/share/zoneinfo;  mkdir -p Africa  America/Argentina  America/Indiana  America/Kentucky  America/North_Da
kota  Antarctica  Arctic  Asia  Atlantic  Australia  Etc  Europe  Indian  Pacific US Mexico Chile Canada Brazil)
for f in `cat zonefiles`; do  install   -o root -g wheel -m 444  /usr/obj/usr/src/amd64.amd64/share/zoneinfo/buil
ddir/${f} /usr/share/zoneinfo/${f};  done
install: /usr/obj/usr/src/amd64.amd64/share/zoneinfo/builddir/Africa/Abidjan: No such file or directory
*** [install-zoneinfo] Error code 71
(etc.)

And indeed, I don’t have the /usr/obj/usr/src/amd64.amd64/share/zoneinfo/builddir/ directory.
So I don’t know why it is not builded.
This is a patch to add a double check (so not very clean because we didn’t fix the root cause)

[philip]

I don't like this. It further hides the real problem.

I think builddir is created by the zonefiles target. I've run into this before but I've never had a chance to try to reproduce it reliably. I'll try to find time to take a closer look.