(Mostly) fix building bin/tests/system/rpz/dnsrps

Building the bin/tests/system/rpz/dnsrps helper binary is currently not
possible at all as the necessary compiler and linker flag definitions
are missing from bin/tests/system/Makefile.am.  Add these as a basis for
addressing the problem.

Unfortunately, this is where the "mostly" bit mentioned in this commit's
subject line comes into play.  The dlopen() parts of DNSRPS code have
not yet been reworked to use libuv's dlopen() API (uv_dlopen() etc.)
(See commit 37b9511ce1 for prior work in
this area.)  While it is certainly possible to do that, implementing
such a change without testing it in practice against a usable librpz.so
(i.e. a DNSRPS provider library) is bound to cause more trouble and
confusion than keeping the code the way it is right now.  However,
making that code buildable as-is requires linking against a C standard
library that exports the dlopen(), dlsym(), and dlclose() symbols used
by the DNSRPS dynamic loading code.  glibc 2.34+ satisfies that
requirement, but older glibc versions do not (these come with a separate
libdl shared library that would need to be linked in as well).  (Other
C standard library implementations have not been examined.)  Since the
long-term plan is to rely on libuv's dlopen() API exclusively and
detecting the shared object containing dlopen() & friends would only
pull in build system complexity for no good reason, assume for now that
the target system provides the dlopen() API in its C standard library.

This change enables the system test suite to be run for a BIND 9 build
prepared using --enable-dnsrps --enable-dnsrps-dl (on systems satisfying
the requirement explained above).  However, it is important to note that
this change by itself does NOT enable actual testing of the DNSRPS
feature as doing that requires a DNSRPS provider library to be present
on the test host.

bin/tests/system/Makefile.am

@@ -48,6 +48,14 @@ pipelined_pipequeries_LDADD =	\
 	$(LDADD)		\
 	$(LIBDNS_LIBS)
 
+rpz_dnsrps_CPPFLAGS =		\
+	$(AM_CPPFLAGS)		\
+	$(LIBDNS_CFLAGS)
+
+rpz_dnsrps_LDADD =		\
+	$(LDADD)		\
+	$(LIBDNS_LIBS)
+
 tkey_keycreate_CPPFLAGS =	\
 	$(AM_CPPFLAGS)		\
 	$(LIBDNS_CFLAGS)