The invention relates to compounds of formula (I) wherein the substituents X¹, R¹, R², R³ and R⁴ have the meaning as set forth and explained in the description of the invention, to processes for the preparation of these compounds, pharmaceutical compositions containing same, the use thereof optionally in combination with one or more other pharmaceutically active compounds for the therapy of a disease which responds to an inhibition of protein kinase activity, and a method for the treatment of such a disease.