Clean up the m4 to handle the auto case always and be more consistent.
Also simplify the CPP --- we should always have both headers if we are
using libnuma.

"side effects" (AC_DEFINE, and AC_SUBST) are removed from the macros to
better separate searching from actions taken based on search results.
This might seem overkill now, but will make shuffling logic between
configure scripts easier later.

The macro comments are converted from `dnl` to `#` following the
recomendation in
https://www.gnu.org/software/autoconf/manual/autoconf-2.71/html_node/Macro-Definitions.html