On non-Windows, hard depend on uselocale(3)

I have been working on adding using thread-safe locale APIs within
Postgres where appropriate[0]. The patch that I originally submitted
crashed during initdb (whoops!), so I worked on fixing the crash, which
led me to having to touch some code in chklocale.c, which became
a frustrating experience because chklocale.c is compiled in 3 different
configurations.

> pgport_variants = {
>   '_srv': internal_lib_args + {
>     'dependencies': [backend_port_code],
>   },
>   '': default_lib_args + {
>     'dependencies': [frontend_port_code],
>   },
>   '_shlib': default_lib_args + {
>     'pic': true,
>     'dependencies': [frontend_port_code],
>   },
> }

This means that some APIs I added or changed in pg_locale.c, can't be
used without conditional compilation depending on what variant is being
compiled. Additionally, I also have conditional compilation based on
HAVE_USELOCALE and WIN32.

I would like to propose removing HAVE_USELOCALE, and just have WIN32,
which means that Postgres would require uselocale(3) on anything that
isn't WIN32.

[0]: https://www.postgresql.org/message-id/CWMW5OZBWJ10.1YFLQWSUE5RE9@neon.tech

--
Tristan Partin
Neon (https://neon.tech)