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 such support is better added to bioformats.git `cpp/cmake/CompilerChecks.cmake` if we want it in-tree, but it should support all compilers including MSVC and have the appropriate functional checks as we do for the other compiler flags.  Happy to do this if you like.