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.
Is this sufficiently portable for compilers without these flags?  For the existing set, we're explicitly checking the compiler supports them before using them.  Does it make MSVC choke?  Does GCC error out for unsupported flags?  (flag support is also GCC version-specific)