Fix contrib/sepgsql and contrib/xml2 to always link required libraries.

contrib/xml2 can get by without libxslt; the relevant features just
won't work.  But if doesn't have libxml2, or if sepgsql doesn't have
libselinux, the link succeeds but the module then fails to work at load
time.  To avoid that, link the require libraries unconditionally, so
that it will be clear at link-time that there is a problem.

Per discussion with Tom Lane and KaiGai Kohei.

contrib/sepgsql/Makefile

@@ -18,5 +18,5 @@ include $(top_builddir)/src/Makefile.global
 include $(top_srcdir)/contrib/contrib-global.mk
 endif
 
-SHLIB_LINK += $(filter -lselinux, $(LIBS))
+SHLIB_LINK += -lselinux
 REGRESS_OPTS += --launcher $(top_builddir)/contrib/sepgsql/launcher

contrib/xml2/Makefile

@@ -8,7 +8,7 @@ DATA = xml2--1.0.sql xml2--unpackaged--1.0.sql
 
 REGRESS = xml2
 
-SHLIB_LINK += $(filter -lxslt, $(LIBS)) $(filter -lxml2, $(LIBS))
+SHLIB_LINK += $(filter -lxslt, $(LIBS)) -lxml2
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config