The logic in commit c96fe0b4 (cmake: Add -W options to control
deprecation warnings and errors, 2015-07-28) accidentally enables
errors on warnings by default in scripts because there is no
initialization of CMAKE_SUPPRESS_DEVELOPER_ERRORS to TRUE.

Rename internal CMAKE_SUPPRESS_DEVELOPER_ERRORS cache entry to
CMAKE_ERROR_DEVELOPER_WARNINGS.  Fix the logic in the message() command
to treat AUTHOR_WARNING as an error only if the option is explicitly
enabled.