Prefix QuEST CMake options

[vaferreiQMT]

Currently QuEST's CMake variables, such as BUILD_EXAMPLES and ENABLE_TESTING are defined globally with no prefix. This is fine when used in isolation but can become a bit of a hassle when QuEST is used as a dependency/ within a larger project.
These options should be prefixed to avoid clashes (e.g. QUEST_BUILD_EXAMPLES).

[otbrown]

Hi Vasco -- no argument from me! I've plans to revisit the MPI in QuEST over the next couple of weeks, so I can create a PR for this at the same time if you haven't already.