Enable to build with ninja

[uk-ar]

Some cmake generators like Ninja cannot handle implicitly generated files, failing with messages like (in case of Ninja):

ninja: error: 'mruby/host/lib/libmruby.a', needed by 'h2get', missing and no known rule to make it

The solution is just to define files with BYPRODUCTS libmruby.a libonigumo.a in ADD_CUSTOM_TARGET. With this patch, cmake -G Ninja -Bb . && cmake --build b works!

See also https://github.com/h2o/h2o/pull/2504