configure: Place all temporary files in one separate directory

Place all temporary files within a single, quasi-atomically created
temporary directory rather than relying on unsafe 'mktemp -u'. This
prevents possible race conditions in case two parallel 'mktemp -u' calls
returned the same path. Additionally, it reduces TMPDIR pollution by
keeping all test files in a single subdirectory.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
