don't use C99 static array indices with older GCC versions

For example, the FreeBSD GCC (4.2.1) has a spotty support for that
feature. If the static keyword is used with an unnamed array parameter
in a function declaration, then the compilation fails with:
error: static or type qualifiers in abstract declarator

The feature does work if the parameter is named.
So, the restriction introduced in this commit can be removed when all
affected function prototypes have the workaround.

MFC after: 1 week
Sponsored by: Panzura