Revert the hack and the original faulty commit.
This reverts commit 48264ee506105a2f5e61e5d327599e9f301bd77f.
Revert "Purity checking should accept $TMP and not just /tmp"
This reverts commit fb777be7d2.
I hate the thing too even though I made it, and rather just get rid of
it. But we can't do that yet. In the meantime, this brings us more
inline with autoconf and will make it slightly easier for me to write a
pkg-config wrapper, which we need.
