Use AC_CHECK_TYPES and AC_CHECK_MEMBERS.

Use AC_CHECK_TYPES and AC_CHECK_MEMBERS.

Rather than rolling our own autoconf macros to test for the existence of
data types and structure members, use AC_CHECK_TYPES and
AC_CHECK_MEMBERS.

That cleans the configure script code up a bit, and makes it a bit more
like the CMakeLists.txt file.