Reinstate use of compatibility functions for macOS versions that need them

[ryandesign]

3697c74 removed the use of compatibility functions such as utimensat which ensures that tup no longer builds on older macOS versions that don't have those functions. Please reinstate the use of the compatibility functions for older OS versions that need them.

[gittup]

What version(s) of macOS are you referring to? IIRC, I removed those compatibility functions once the last version of macOS that needed them was no longer supported, but I could be mistaken.

[ryandesign]

"Supported" is nebulous. Supported by whom?

utimensat was introduced in macOS 10.13 so a compatibility function is needed for macOS 10.12 and earlier.