I came across this while trying to work around #661 (I'm on a Mac, building a native app). When building firefox, we force-include /mozilla-config.h, which has a significant number of #defines (around 150 by my count right now). As it stands, I can copy/paste them from my mozilla-config.h, but this file is auto-generated, and subject to change (at least once per firefox release, every 6 weeks), so that's not the most useful or practical thing to do in the long run. Our command line for building with clang includes -include /Users/nwgh/src/mozilla/gecko/obj-dbg/mozilla-config.h. It would be nice to have this same effect in c_cpp_properties.json without having to copy/paste and double-check my work every once in a while.

I'll also note that adding the defines from that file does not actually fix #661 for me, but from reading that issue, it at least seems like the appropriate first step.