Handle floorl() the same way we handle other not-on-all-platforms functions.

Use AC_CHECK_FUNC() for it, define FLOORL_LO to floorl.lo if we *don't*
have it, add FLOORL_LO to the list of items conditionally built in
libwsutil, and include "wsutil/floor.h" only if HAVE_FLOORL is *not*
defined, as that means it's *not* supplied by the platform and thus
*not* declared in <math.h>.

Also, use the standard export stuff in wsutil/floor.h.

Change-Id: Ic24aa69f65f2d15450d8b84b0c2b0c58f38edebe
Reviewed-on: https://code.wireshark.org/review/5901
Reviewed-by: Guy Harris <guy@alum.mit.edu>