configure.ac: Look in the right place for `socklen_t'.

The old-fashioned version of `AC_CHECK_TYPE' doesn't look in
<sys/socket.h>, which is the proper place.  Android doesn't appear to
define the type in the usual places where Autoconf looks.

So use the modern version of the check and look in <sys/socket.h>
explicitly.  This seems to fix the build on Android.