Enable improved <tgmath.h> for Clang even when not using C11.
Enable improved <tgmath.h> for Clang even when not using C11.

By using __has_extension(c_generic_selections), we can explicitly test
whether we're dealing with a version of Clang that supports _Generic().
That way we can use the improved <tgmath.h> code, even when not using
-std=c11. This massively reduces the compilation time when invoking
these functions.

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