Better support for C++ projects with precompiled headers

[bernardnormier]

If your C++ project uses a precompiled header, your best choice today is to add the rather obscure option --add-header <pch file> in the Additional Options of your Slice compilation. With this option, the precompiled header is included at the top of the generated C++ source file.

It would be nice if this support was more automatic. For example, Ice Builder could disable precompiled header file support when C++-compiling the generated C++ source files. Or alternatively, Ice Builder could offer a new SliceCompile item metadata for precompiled header support, mapped to --add-header.