Delete gcc dependency.

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The system such like FreeBSD, no more use gcc as default. So the compiler must specify no gcc directly but $(CC) macro.

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This breaks all sorts of systems whose base compiler doesn't support all the GNU command lines or extensions. The Makefiles force gcc for a good reason as they do not stick to portable compiler options or features.

If you want to allow for clang then it needs to actually detect and verify if gcc is present and if not then check if clang is present.