This change uses the `find_program(VALIDATOR)` functionality available
since CMake 3.25 to find the acceptable Bison executable instead of
failing if the first one found is too old. (macOS ships with Bison 2.6.)
This change also explicitly finds a usable M4 and ensures the Bison
command will use it. (macOS ships with an m4 which can fail if XCode
Tools aren't installed.)
