Use macOS specific directory instead of /tmp

[stweil]

In addition, a trivial compiler warning was fixed by using an ANSI C function prototype.

[stweil]

These modifications were inspired by pull request #709 which is replaced here. All tests pass on macOS:

============================================================================
Testsuite summary for leptonica 1.84.0
============================================================================
# TOTAL: 146
# PASS:  146
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

[DanBloomberg]

Thank you. Really nice to have this resolved.
Good to see the test suite passes on macOS.
All looks good except for comments at lines 2395 and 2345.

[stweil]

All looks good except for comments at lines 2395 and 2345.

Please help me. How should those comments look like?

[DanBloomberg]

Not a big deal; can be modified later.