Simplify CMake configuration

[wonder-sk]

There are a few things that would make things simpler when setting up a build with CMake:

• USE_MM_SERVER_API_KEY - this should be FALSE by default, so it can be omitted
• QGIS_QUICK_DATA_PATH - this should have a default path, so it can be omitted
• INPUT_SDK_PATH - it would be good to check if the passed directory exists - sometimes I pass a wrong directory and then get a random looking configuration error

Bonus points for a check whether the SDK is compatible (we could have a file in SDK package indicating the version, and we could check that against what CI currently expect) - it would help if we quickly new we're using outdated mobile SDK locally 🙂