Reduce Ice-generated C++ warnings

Rather than simply listing (and ignoring) all the warnings
that come with ZeroC-generated code we now suppress their
output at the cmake level.

In addition, using `cmake -Dfatal-warnings=true .`
will make all warnings an error which forces cleaning
up these issues.

/cc @rleigh-dundee @sbesson -- suggestions on better header names welcome.
Note that you could check that the options are in the supported set.  `list(FIND` or `list(GET` would be possibilities.  The former could be looped over for each flag; the latter will search for multiple flags in a single shot.