Remove redundancy in cmake file

[markfoodyburton]

Assume that there is always the top level cmake file, dont try to have 'standalone' cmakefile in each sub-directory.

[mmillieri]

I have looked at this pretty deeply now. I'm now not convinced that this is the right thing. The annoying thing about the current structure, is that it always wants to install SystemCLanguage and won't install the "header-only" interface libraries (i.e. the tlm_extensions) locally (always wants to put it in CMAKE_INSTALL_PREFIX).

I think we should have another (in my case, more informed) discussion on whether CMakeLists.txt files that each can be "top level" may be a pretty good solution for "pick and choose" consumers.