README: -DPOLYBENCH_USE_C99_PROTO should only be used on ppcg command line
README: -DPOLYBENCH_USE_C99_PROTO should only be used on ppcg command line

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

nicolasvasilache/ppcg,Meinersbur/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg