Make ISC rwlock implementation the default again

Revert the change from ad03c22e97 as
further testing has shown that with hyper-threading disabled, named with
ISC rwlocks outperforms named with pthread rwlocks in cold cache testing
scenarios.  Since building named with pthread rwlocks might still be a
better choice for some workloads, keep the compile-time option which
enables that.

configure

@@ -1624,9 +1624,8 @@ Optional Features:
   --enable-devpoll        use /dev/poll when available [default=yes]
   --disable-geoip         support GeoIP2 geolocation ACLs if available
                           [default=yes]
-  --disable-pthread-rwlock
-                          use internal rwlock implementation instead of
-                          pthread rwlock
+  --enable-pthread-rwlock use pthread rwlock instead of internal rwlock
+                          implementation
   --enable-fips-mode      enable FIPS mode in OpenSSL library [default=no]
   --enable-native-pkcs11  use native PKCS11 for public-key crypto [default=no]
   --disable-tcp-fastopen  disable TCP Fast Open support [default=yes]
@@ -16001,7 +16000,7 @@ done
 if test "${enable_pthread_rwlock+set}" = set; then :
   enableval=$enable_pthread_rwlock;
 else
-  enable_pthread_rwlock=yes
+  enable_pthread_rwlock=no
 fi
 
 

configure.ac

@@ -685,9 +685,9 @@ AC_SUBST(ALWAYS_DEFINES)
 # Do we want to use pthread rwlock?
 #
 AC_ARG_ENABLE([pthread_rwlock],
-	      [AS_HELP_STRING([--disable-pthread-rwlock],
-			      [use internal rwlock implementation instead of pthread rwlock])],
-	      [], [enable_pthread_rwlock=yes])
+	      [AS_HELP_STRING([--enable-pthread-rwlock],
+			      [use pthread rwlock instead of internal rwlock implementation])],
+	      [], [enable_pthread_rwlock=no])
 
 AS_IF([test "$enable_pthread_rwlock" = "yes"],
       [AC_CHECK_FUNCS([pthread_rwlock_rdlock], [],