When using Clang with Visual Studio, the GenerateExportHeader function
generates the DEPRECATED definition "__attribute__ ((__deprecated__))",
because Clang can compile such instruction. However, if a user wants to compile
the library with this generated header with MSVC, the declaration is not valid.
We suggest to always use the DEPRECATED definition "__declspec(deprecated)" on
Windows, which is valid for both Clang and MSVC.
