Unfortunately, it's quite inconvenient to use precompiled, there are a lot of limitations in each compiler. For instance, only the first header include a source file can be precompiled and other things like that, this is a more general approach. But PCH can bring a really good speedup too and are free ;)
This was my question too. Zapcc's faq says: Precompiled headers requires building your project to the exact precompiled headers rules. Most projects do not bother with using precompiled headers. Zapcc works within your existing build.
Precompiled headers are currently ignored by Zapcc.
http://clang.llvm.org/docs/UsersManual.html#precompiled-head...