For XS I have only one single definite guideline: use Devel::PPPort!

For all other things regarding XS, you'll have to rely on CPANTESTERS for all architectures that you do not have access to.

Furthermore, you should not use any C feature that falls outside of the requirements of perl itself. Perl requires C89 C standards, so if you want to use C99 standards, you're likely to fail on (older) systems that do not have C99 compilers available. C99 features also include the dreaded use of C++ style comments, so don't use them. Ever. (Yes, I know that all compilers on windows systems support this awful comment style, so it is very tempting for developers on windows to use it.)