Tidy the cmake config and add better build configuration support.

[dbaarda]

This is an idea taken from the following post;

https://blog.kitware.com/cmake-and-the-default-build-type/

It sets the default build type to Debug for git clone, otherwise the default is Release. It also sets possible values for the cmake-gui or ccmake. It works with multi-config generators and respects values set on the cmdline.

[dbaarda]

I have some mild concerns about this related to the following comment;

https://blog.kitware.com/cmake-and-the-default-build-type/#comment-8726

I don't know who might be affected by this though. Surely distro's relying on building using the default empty build-type have a workaround for projects using this approach.

I feel like the benefits of using this outweigh the risks. It means most naive users doing their own builds will end up with a Debug build with trace turned on, improving the chance they will detect problems. The improved cmake-gui and ccmake build-type selection will also make it easier for them to select a Release build when they realize they've been using a Debug build. Outputting the build-type in a cmake message makes it easier for them to realize/verify what build-type they are using.

Advanced users will already be setting CMAKE_BUILD_TYPE on the cmake cmdline, and this will not affect them.

I did try experimenting with supporting a build type of "" in the selection options but it doesn't work. The empty option disappears from the selection list, and setting -DCMAKE_BUILD_TYPE="" is the same as not setting it at all from the point of view of detecting/setting the default to Debug. It is possible but a bit ugly to add a "empty" build type that can be selected on the cmdline and saved in the cmake cache that then clears CMAKE_BUILD_TYPE, but it's a bit ugly.

So I think I'm going to submit this and we'll address any problems later if they surface.