c.m4: Use __has_attribute to check for attribute availability.

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This fixes -Wunused-but-set-variable warnings on clang msvc targets. Commit
a166d5450e3f47587b98f6005f9f5627dbe21a5b fixed those warnings in most 
cases,
but currently YY_ATTRIBUTE_UNUSED is defined only when __GNUC__ is defined.
When using clang in msvc mode, it's not defined, but __unused__ attribute
is still available.


In my use case, it fixes warnings when building the Wine project with 
clang as

a cross compiler.

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