Hide properties type on FreeBSD.

In `perl.h`, there is:
```
#ifdef I_LIBUTIL
#include
#endif
```
and on FreeBSD, `I_LIBUTIL` is defined in Perl's `config.h`. That header does
not really appear to be necessary for Perl, but it has the side effect of
declaring a `properties` type. This is a generic name used in our code base, so
we hack around it with the preprocessor. Preferably, FreeBSD would fix that in
their Perl port in the future.