Use mktemp for more robust temporary directory creation

[samcmill]

uuidgen was missing on a cluster and didn't notice it since configure was "successful". As a result, parallel makes (make -j) were failing because WRKDIR was not unique for each process.

Rather than construct a "unique" temporary directory name by hand, just use mktemp.

Also, updated configure to stop with an error if either mktemp or uuidgen is not found.

[rtmclay]

Thanks very much for the patch!