As some users may have installed GNU coreutils on Mac OS X, e.g. `brew install coreutils`, it's possible that
the `date` command uses the GNU version, instead of the standard BSD version.
- Detect if GNU coreutils is installed on Mac and if it is, use the GNU variants of date functions
- Reduce some of the duplication in the file, which resolves a bug where the cygwin version of `how_many_days_from_today()`
had the operands switched around, leading to a positive result instead of negative
- Add test_tcp_connectivity function for cygwin (uses the GNU variant)
Fixes#534
