progdefs.h in FTEQCCGUI

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qcc needs to generate it in order to get the correct progs api crc. If you use a custom one in an engine, then your engine is incompatible with every other mod. So unless you want to make an engine that doesn't support running any mods, generating a progdefs.h (at least writing one to disk, anyway) is entirely useless.
If you want to see how you messed up your defs.qc then I can see a point. But you can just compare your defs.qc instead of progdefs.h, so no real gain.
But yeah, you can still write one with that extra commandline argument.