Linux install

@dgirardeau  here are some changes for installing on linux, but I think it is still not complete.
- It is not clear to me why to keep both debug and standard executables and providing different install dirs....
I normally compile cloudcompare as RelWithDebInfo, but the normal user would simply compile the Release version. 

- I tried to compile it with clang on linux. I made some changes for being able to compile it.

Feel free to not accept those changes. I am not sure if it is a good work. 

BTW one day we should think to install also a CloudCompareConfig.cmake file so that e.g. external tools may begin to use CClib as computational backend + some header files so that one may be able to develop external plugins without to be forced to work from within the cc codebase.