liblp: Use TMPDIR instead of P_tmpdir.

lpmake should be using the intermediates directory for temporary work
rather than /tmp.  Add ability to respect TMPDIR environment as
inherited from TemporaryFile.

Bug: 119313545
Test: manual test
Change-Id: I1a0317538875ee37fb4066602ff7a75e4658d74b
1 file changed