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.
To add on the command-line?  Just set `CXXFLAGS` in the environment or set on the command-line with `-DCMAKE_CXX_FLAGS="<FLAGS>"`.  The former is recommended; the latter may override platform-specific defaults (which is useful, but not normally desired) which is mainly an issue on Windows since with GCC/clang it usually only needs -g/-O2 as desired.