Prevent perl header overriding our *snprintf macros, and give it a usable PERL_UNUSED...

Prevent perl header overriding our *snprintf macros, and give it a usable PERL_UNUSED_DECL value.

This quiets compiler warnings about redefined macros and unused Perl__unused variables. The
redefinition of snprintf and vsnprintf is something we want to avoid anyway, if we've
gone to the bother of setting up the macros to point to our implementation.