GenerateExportHeader: Support C++ 14 [[deprecated]] attribute.
Since C++ 14 there is a standard way of deprecating APIs in the form of the [[deprecated]]
attribute. This attribute also supports specifying a deprecation message.
It would be nice to support the latter in the GenerateExportHeader
module. Besides the MYLIB_DEPRECATED
macro, we would have MYLIB_DEPRECATED(message)
macro that would expand to [[deprecated(message)]]
on C++14 or greater.