fix: don't define _POSIX_C_SOURCE on FreeBSD

Curiously this was breaking compilation on FreeBSD by hiding some type
definitions. Casting around on the internet to debug the problem, most
references led me back to [0]. This seems odd, but apparently the right thing to
do is #define nothing on FreeBSD.

  [0]: https://lists.freebsd.org/pipermail/freebsd-standards/2004-March/000474.html

Github: related to #134 "Tests fail: UnicodeDecodeError: 'ascii' codec can't
  decode byte 0xe2 in position 120: ordinal not in range(128)"