Add libXMU in core dependency tree (CDT)

[matmel]

Hello,

I am trying to build an internal package with the new conda-build 3 with Anaconda compilers. After a lot trial and error, and by reading this ticket, https://github.com/conda/conda-docs/issues/607, I realized I need to find all system RPMs on which my tool is depending on.

Well, after identifying one by one (each time involving a conda build followed by a symbol resolution failure) all the required X libraries, and adding the corresponding - {{ cdt{'name-of-centos6-rpm-found-on-google') }}, I am blocked at libXmu-devel`. I think you didn't package that one. I have no idea on how this black magic works.

While I am at it, is this the expected workflow to identify all the CDT dependencies? It seems very clunky to me... I like the fact that I will be able to compile anything outside CentOS 6 and making sure that my package still works on CentOS 6. That is important for us.