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.
BTW, if you're running into problems, I'd suggest printing out what's going on at each step with e.g. `message(STATUS "FLAGS: ${ICE_FLAG_LIST}")`.  This should make it fairly simple to check what the state is and what's happening/not happening at each step.