Turn off warnings coming from subprojects
If you're using Xcode 4.2 or later, you can use LLVM compiler setting Inhibit All Warnings in Warning Polices for every subproject you have:
A partial solution only:
If your subprojects build libraries or bundles that do not have to recompile at every run, you can activate the "recent filer" (see figure, blue clock). Then, only warnings and errors from the last compiler run are displayed.
If the library isn't too big, You could disable warnings on a per-file basis using
#pragma GCC diagnostic ignored "-Wwarning-flag"