cmake warns when mariadb is compiled without mysql_release

When maridb is built without specifying BUILD_CONFIG=mysql_release the compilation comment in mysql_version.h is not set to be MariaDB Server. It stays with the default Source distribution (set in mariadb:cmake/mysql_version.cmake).

MaxScale's cmake checks the compilation comment in cmake/FindMySQL.cmake and spews a warning:

-- MySQL version: 10.0.17

-- MySQL provider: Unknown

CMake Warning at cmake/FindMySQL.cmake:33 (message):

  Not using MariaDB server.

Call Stack (most recent call first):

  CMakeLists.txt:28 (find_package)

I think the build then continues successfully, but this warning is quite confusing.