androidCompileSdkVersion cannot be set from CMake alone

When building for Android with CMake, the compile SDK version can only be set when building with Qt Creator by modifying the "Build Android APK" build step.

When building from the command line, however, there is no CMake option / target property that allows to specify this SDK version. Instead, androiddeployqt always chooses the latest available SDK, which might not always be the desired one.

Possible fix: Add a CMake target property alongside QT_ANDROID_MIN_SDK_VERSION and QT_ANDROID_TARGET_SDK_VERSION, for example QT_ANDROID_COMPILE_SDK_VERSION.