glxspheres: handle new SDL platform enumerations
vogl_replay_tool: return NULL instead of false
cmake: check for policies properly

Instead of hard-coding the version, check for the policy directly. If
anyone is using a 2.8.11 pre-release version (though this would be…odd),
let it work for them as well.
cmake: avoid warnings with CMake 3.0

Policy CMP0046 disallows adding non-target dependencies to targets.
Check if SDL is a target before adding the dependency.
cmake: use generator expression for testing

Policy CMP0026 deprecates the LOCATION property. Instead, use a
generator expression to get the location of the output of the target.
This feature was added in 2.8.0, so no version bump is required.
Merge pull request #100 from mathstuf/misc-fixes

Misc fixes