Clean up preprocessor directives and some obsolete cmake options.

 * Make use of built in defines VALA_X_Y.
 * Use LIB_X_Y instead of LIB_LESS_X_Y (now >= X_Y instead of < X_Y).
 * Output some warnings in special not recommended library version situations.
 * Fix gdl-3.0 <= 3.6.2 and vala <= 0.20 issue.