configure: rework libXinerama detection a bit, remove hardcoded -lXinerama's

Make -lXinerama a dep on libDtXinerama so it doen't need to be
specified in Makefile.am files or in LIBS as it was being done
previously.

This still needs a little work, ideally Xinerama should be completely
optional and only enabled if present.  But we can save that for
later.

cde/programs/dtmail/dtmailpr/Makefile.am

@@ -5,8 +5,7 @@ bin_PROGRAMS = dtmailpr
 dtmailpr_CXXFLAGS = -I../include -I../../dtcompat -I../../../. \
 		    -I$(srcdir)/lib
 
-dtmailpr_LDADD = ../libDtMail/libDtMail.a @DTCLIENTLIBS@ \
-		 -lXm @LIBXIN@ -lXinerama $(XTOOLLIB) ${X_LIBS} $(TIRPCLIB)
+dtmailpr_LDADD = ../libDtMail/libDtMail.a $(DTCLIENTLIBS) $(XTOOLLIB)
 
 if FREEBSD
 dtmailpr_LDADD += -liconv