dl: need to use LIBADD, not LIBS

When we use LIBADD for static libraries, the dependent libraries get
propagated properly.  For example, the dl/dlopen component will almost
certainly require the -ldl library; when using LIBS, that doesn't get
propagated elsewhere in the tree, but when using LIBADD, it does
(e.g., when linking opal_wrapper_compiler).

opal/mca/dl/dlopen/Makefile.am

@@ -20,4 +20,4 @@ noinst_LTLIBRARIES = libmca_dl_dlopen.la
 
 libmca_dl_dlopen_la_SOURCES = $(sources)
 libmca_dl_dlopen_la_LDFLAGS = -module -avoid-version
-libmca_dl_dlopen_la_LIBS = $(opal_dl_dlopen_LIBS)
+libmca_dl_dlopen_la_LIBADD = $(opal_dl_dlopen_LIBS)

opal/mca/dl/libltdl/Makefile.am

@@ -23,4 +23,4 @@ libmca_dl_libltdl_la_CPPFLAGS = $(opal_dl_libltdl_CPPFLAGS)
 libmca_dl_libltdl_la_LDFLAGS = \
         $(opal_dl_libltdl_LDFLAGS) \
         -module -avoid-version
-libmca_dl_libltdl_la_LIBS = $(opal_dl_libltdl_LIBS)
+libmca_dl_libltdl_la_LIBADD = $(opal_dl_libltdl_LIBS)