Make the build reproducible.

[lamby]

Whilst working on the Reproducible Builds effort [0], we noticed
that fswatch could not be built reproducibly.

This is because the manpage used the output of the date(1) cmd.

This has been filed in Debian as #882818 [1].

[0] https://reproducible-builds.org/
[1] https://bugs.debian.org/882818

[emcrisostomo]

Thanks @lamby. That command would fail when running a date implementation that doesn't support those options (--utc and --date, such as BSD date) so we should probably add some check before running that command. I'll be tracking it in a separate issue though. In the meantime I'm merging this PR.