Add -Wold-style-definition to CFLAGS?

Hi,

ISTM that it's our coding style that we use

something
my_paramless_func(void)
{
...
}

definitions rather than omitting the (void), which makes the function
look like an old-style function declaration. I somewhat regularly notice
such omissions during review, and fix them.

Since gcc has a warning detecting such definition, I think we ought to
automatically add it when available?

The attached patch makes configure do so, and also fixes a handful of
uses that crept in.

Greetings,

Andres Freund