In commit v3.4.0-rc1~480^2~3 (cmCustomCommandGenerator: Port to
cmOutputConverter, 2015-06-04), cmCustomCommandGenerator's access to the
local generator was removed so it needed to construct its own
cmOutputConverter instance.  Access to the local generator was then
restored by commit v3.4.0-rc1~285^2~21 (cmCustomCommandGenerator:
Require cmLocalGenerator in API, 2015-07-25), so now we can use its
cmOutputConverter base class methods directly.