[master] (re)add the LIBRARY_PATH_PLACEHOLDER trick for dhcp6 tests

now its python test relies on isc python modules we need this workaround
for MacOS X.  This fix solves recent buildbot failures on macmini.
okayed on jabber

src/bin/dhcp6/tests/Makefile.am

@@ -2,10 +2,18 @@ PYCOVERAGE_RUN = @PYCOVERAGE_RUN@
 PYTESTS = dhcp6_test.py
 EXTRA_DIST = $(PYTESTS)
 
+# If necessary (rare cases), explicitly specify paths to dynamic libraries
+# required by loadable python modules.
+LIBRARY_PATH_PLACEHOLDER =
+if SET_ENV_LIBRARY_PATH
+LIBRARY_PATH_PLACEHOLDER += $(ENV_LIBRARY_PATH)=$(abs_top_builddir)/src/lib/cryptolink/.libs:$(abs_top_builddir)/src/lib/dns/.libs:$(abs_top_builddir)/src/lib/dns/python/.libs:$(abs_top_builddir)/src/lib/cc/.libs:$(abs_top_builddir)/src/lib/config/.libs:$(abs_top_builddir)/src/lib/log/.libs:$(abs_top_builddir)/src/lib/util/.libs:$(abs_top_builddir)/src/lib/exceptions/.libs:$(abs_top_builddir)/src/lib/util/io/.libs:$(abs_top_builddir)/src/lib/datasrc/.libs:$$$(ENV_LIBRARY_PATH)
+endif
+
 # test using command-line arguments, so use check-local target instead of TESTS
 check-local:
 	for pytest in $(PYTESTS) ; do \
 	echo Running test: $$pytest ; \
+	$(LIBRARY_PATH_PLACEHOLDER) \
 	PYTHONPATH=$(COMMON_PYTHON_PATH):$(abs_top_srcdir)/src/bin:$(abs_top_builddir)/src/bin/bind10:$(abs_top_builddir)/src/lib/util/io/.libs \
 		$(PYCOVERAGE_RUN) $(abs_srcdir)/$$pytest || exit ; \
 	done