Unbreak the build on PowerPC. This is a quick fix rather than perfectly
correct: REGINA_BITS macro is set by configure script (for known systems,
but passed to the compiler unconditionally even if empty) and checked and
set in `rexx.h' as well if defined(APPLE) && defined(MACH).

Better approach would be either making configure script logic exhaustive,
or move REGINA_BITS setting entirely into `rexx.h', leaving the ability
to override it via --enable-{32,64}bit configure arguments.