Add a new message type for deprecation.

By default, the message is not issued. If CMAKE_ERROR_DEPRECATED
is on, the message is fatal. If CMAKE_WARN_DEPRECATED is on, the
message is a warning.
Port Qt4Macros to the new DEPRECATION message.
Merge topic 'deprecation-message' into next

3e781fc Port Qt4Macros to the new DEPRECATION message.
d700e3b Add a new message type for deprecation.