Let us enable all GCC and clang warnings independently of the
MYSQL_MAINTAINER_MODE setting for both Debug and RelWithDebInfo
builds, and have MYSQL_MAINTAINER_MODE only enable -Werror.

The default setting of MYSQL_MAINTAINER_MODE=AUTO will continue
to apply the -Werror only to CMAKE_BUILD_TYPE=Debug. To build
a debug version without -Werror, MYSQL_MAINTAINER_MODE=OFF can
be used.