Issue Description:

Recently swift-build was fixed so that this warning is properly printed out:

'sdl2' sdl2.pc: warning: non whitelisted flag(s): -D_THREAD_SAFE

However, I do not think that this is sufficient. To me, since this is just a warning and compilation continues, my first assumption was that this individual flag was ignored, but all the other flags from pkgconfig were retained. I eventually had to build SPM myself and add some debug print statements to figure out that my build was failing because this warning means that all flags from the pkgconfig were dropped, for both building and linking.

For my use case, I'd prefer if the behavior was to just print the warning, drop those specific flags, and retain the rest of the pkgconfig flags. But if that's not an option, this warning should be made more clear about the fact that all flags have been dropped, not just these specific flags, or perhaps compilation should just be halted.

(The fact that this flag isn't whitelisted is a separate issue already tracked in SR-7689).