Some improvements and cleanup of CMake build

[fpagliughi]

I've started working on some updates to the CMake build with the following goals:

1. Upgrade the targeted version of CMake to v3.5. It seems that earlier versions are being deprecated and causing build warnings with current versions of CMake. (See Bumped CMake minimum to 3.5 and added a CMake project version #1425)
2. Set the CMake version of the project so that downstream users can request/check a minimum version in their CMake builds. (See Bumped CMake minimum to 3.5 and added a CMake project version #1425)
3. Make it easier to use Paho C as a submodule in other projects. A few minor things - like aliasing the namespace - would make it easy to include the whole project as a subdirectory in CMake.
4. Do a general cleanup of the CMake files to make them more consistent and readable.
5. Everything should be backward compatible - no breaking changes.

I'm interested in any other ideas people might have to make this easier to use in their projects, especially if anyone is maintaining the this code in a package manager and needs to add any work-arounnds or patches to get things compiling.

[icraggs]

Thanks. The PR has been merged into the develop and 1.4 branches.