the main effect of this is to make GCC and other libiberty using
tools use /tmp instead of /var/tmp for compiler temp files,
which can be a bottleneck on larger systems.
a survey of other platforms shows only OSX also uses /var/tmp,
everyone else has switched to /tmp long ago.
cons: some smaller systems may have a smaller /tmp than /var/tmp,
and this may cause builds to fail with out of space earlier.
point the build to /var/tmp using TMPDIR in this case.
one can argue that setting TMPDIR would work around this, but we
want to have the effect for all users without having special setup.
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