build: Use gcr4 by default, but have an option to use gcr3 too
The gcr4 replaces gcr3, thus make it possible to use either of the two, while defaulting to gcr4, as it'll live for a longer time.
The gcr4 replaces gcr3, thus make it possible to use either of the two, while defaulting to gcr4, as it'll live for a longer time.