later versions of pcc may define __GNUC__ for compatibility reasons, so
having a system definition for alloca is counter productive as it seems
fairly common to use

    #ifdef __GNUC__
    #define alloca ..
    #endif

in user-headers (eg see usr.sbin/gspa/gspa/gspa_ass.h)

So, defang this definition as we don't otherwise provide alloca on NetBSD

(plunky)