Try to find a compiler option that enables usage of char-type
to index a value-field. This one needs unsigned-chars and it
must suppress warnings about usage of chars for subscripting.
for gcc -funsigned-char -Wno-char-subscripts
Currently this macro knows about GCC.
hopefully will evolve to use: Solaris C compiler,
Digital Unix C compiler, C for AIX Compiler, HP-UX C compiler,
and IRIX C compiler.