configuring on openbsd gives the following:

checking pthread_np.h presence... yes
configure: WARNING: pthread_np.h: present but cannot be compiled
configure: WARNING: pthread_np.h:     check for missing prerequisite headers?
configure: WARNING: pthread_np.h: see the Autoconf documentation
configure: WARNING: pthread_np.h:     section "Present But Cannot Be Compiled"
configure: WARNING: pthread_np.h: proceeding with the compiler's result
configure: WARNING:     ## --------------------------------- ##
configure: WARNING:     ## Report this to bind9-bugs@isc.org ##
configure: WARNING:     ## --------------------------------- ##
checking for pthread_np.h... no

this is because the file does not include pthread.h, so the test for it fails.

the following patch fixes it:

diff --git a/configure.in b/configure.in
index 737b1c1..059189b 100644
--- a/configure.in
+++ b/configure.in
@@ -1353,7 +1353,7 @@ then
 
        # Look for functions relating to thread naming
        AC_CHECK_FUNCS(pthread_setname_np pthread_set_name_np)
-       AC_CHECK_HEADERS(pthread_np.h)
+       AC_CHECK_HEADERS([pthread_np.h], [], [], [#include <pthread.h>])
 
        #
        # Look for sysconf to allow detection of the number of processors.