mktemp should be replaced with mkstemp

[Jochem Snuverink]

Compiling with gcc 7.2. I get at linking:

src/BDSTemporaryFiles.cc:58: warning: the use of `mktemp' is dangerous, better use `mkstemp'

https://linux.die.net/man/3/mktemp says "Never use mktemp()."

To get the filename, note that mkstemp will replace the argument given with the filename.

[Laurie Nevay]

Fix Issue #216.

→ <<cset 5f34d2bed659>>