The version check for Zig 0.6.0 was incorrect since commit
971ab7f (Use a zig build script to run ziglings).
Move compatibility support to a separate file, in order to simplify
build.zig.
In case of incompatible version, exit with code 3 instead of 0, in order
to detect the case of failure in a test (to be implemented).
Remove the use of comptime when checking compatibility at the start of
the build function, since it is not necessary.
Closes#210.
