d2x-xl doesn't (directly) use libXm, libXt nor libX11.
Additionally, the way to link C++ apps is with g++, not adding -lstdc++.
Where does dummy.cxx come from?
It's a fake file. It's there just to make automake know that it is linking a C++ program. It comes from automake documentation: http://www.gnu.org/software/automake/manual/html_node/Libtool-Convenience-Libraries.html#Libtool-Convenience-Libraries
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