Support for gdl switcher hacks with new upstream version.

 * New gdl version currently not released (only in master).
 * Switcher hacks are grip showing/hiding and some tab style properties.
Versioned defines guessing preparations.

 * Guess OS platform (Unix/Windows). Set during compile time with CMake.
Fix compilation for valac >= 0.19 and gdl < 3.5.5.