configure: make NTP and ASYNCDNS support nonoptional

Don't allow the NTP support and asynchronous name resolving to be
disabled. pthreads are now a hard requirement.

NTP is the primary task of chrony. This functionality doesn't seem to be
commonly disabled (allowing only refclocks and manual input).

This removes rarely (if ever) used code and simplifies testing.

test/unit/ntp_ext.c

@@ -21,8 +21,6 @@
 #include <config.h>
 #include "test.h"
 
-#ifdef FEAT_NTP
-
 #include <util.h>
 #include <logging.h>
 
@@ -157,11 +155,3 @@ test_unit(void)
 
   }
 }
-
-#else
-void
-test_unit(void)
-{
-  TEST_REQUIRE(0);
-}
-#endif