libclang: declare blocks interfaces always

The implementation of these methods is not reliant on the availability
of the Blocks extension in the compiler.  However, when building on
Windows, the interface declaration is important for the attribution of
the DLL storage.  Without the attribution, the method implementation is
built but not made available as part of the ABI.  Use a check for the
blocks extension to determine how method signature is viewed rather than
controlling whether it is part of the interface.