WL#4650: Allow building the embedded libmysqld as a shared library

Currently, the Embedded MySQL server (libmysqld) is only built as a static
library (libmysqld.a). If possible, it would be useful to also provide a shared
library version (libmysqld.so) that applications could link against dynamically.
That would make it easier to update the embedded server in case of bugs without
having to rebuild the entire application. (See BUG#39288 for more details)