Disable verbose bison output.

These logs aren't generally useful to build everytime, just when working
on and debugging one of the various grammars and at least I haven't
needed to look at them in years.  Also, Ninja builds don't seem to work
because of them (can probably improve the related CMake macros so the
verbose logs do play nice with Ninja, but doesn't seem worth effort
right now, see previous comment).