First we check for C99 support in Makefile.detect, looking for the
header we need and verifying whether it is the right one by using
a definition required by C99 standard to be present in that header.
uintN_t types are optional, but implementations are required to
provide corresponding limit #defines for the types they support,
so we can check for this with preprocessor only.
Finally, we define SEXP_UINTN_DEFINED for any sexp_uintN_t we have
so that the code can use #ifs to check for exact integer support.
