Fix loading plugins using just their filenames

BIND 9 plugins are installed using Automake's pkglib_LTLIBRARIES stanza,
which causes the relevant shared objects to be placed in the
$(libdir)/@PACKAGE@/ directory, where @PACKAGE@ is expanded to the
lowercase form of the first argument passed to AC_INIT(), i.e. "bind".
Meanwhile, NAMED_PLUGINDIR - the preprocessor macro that the
ns_plugin_expandpath() function uses for determining the absolute path
to a plugin for which only a filename has been provided (rather than a
path) - is set to $(libdir)/named.  This discrepancy breaks loading
plugins using just their filenames.  Fix the issue (and also prevent it
from reoccurring) by setting NAMED_PLUGINDIR to $(pkglibdir).

(cherry picked from commit 5065c4686e)

Makefile.tests

@@ -12,7 +12,7 @@ AM_CFLAGS +=					\
 
 AM_CPPFLAGS +=					\
 	$(CMOCKA_CFLAGS)			\
-	-DNAMED_PLUGINDIR=\"$(libdir)/named\"	\
+	-DNAMED_PLUGINDIR=\"$(pkglibdir)\"	\
 	-DSKIPPED_TEST_EXIT_CODE=77		\
 	-DTESTS_DIR=\"$(abs_srcdir)\"
 

lib/ns/Makefile.am

@@ -1,7 +1,7 @@
 include $(top_srcdir)/Makefile.top
 
 AM_CPPFLAGS +=				\
-	-DNAMED_PLUGINDIR=\"$(libdir)/named\"
+	-DNAMED_PLUGINDIR=\"$(pkglibdir)\"
 
 lib_LTLIBRARIES = libns.la