SConstruct : Simplify platform-vs-compiler config
SConstruct : Simplify platform-vs-compiler config

We were specifying `-Wno-unused-local-typedef` in two places, once for Clang and once for MacOS. The new arrangement makes it clearer that the choice of compiler is not necessarily tied to the choice of platform (it's unlikely we'd want to use GCC on MacOS, but it does seem plausible that we'd want to use Clang on Linux).

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