build-sys: allow a build when libdl.so is truly absent

The earlier commit purporting to allow top to be built
in the absence of that dynamic linking library stopped
just a little short of the truth. So this will fix it.

Reference(s):
commit 5686877cd4

Signed-off-by: Jim Warner <james.warner@comcast.net>

top/Makefile.am

@@ -3,7 +3,8 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir)/include \
 	-DLOCALEDIR=\"$(localedir)\"
 
-AM_LDFLAGS = ../proc/libprocps.la -ldl
+AM_LDFLAGS = ../proc/libprocps.la
+AM_LDFLAGS += $(DL_LIB)
 
 if WITH_NCURSES
 usrbin_exec_PROGRAMS = \