Personally i would avoid the use of #ifdef USE_MYSQL - while there are no plans to change it, eventaully i suspect it will go.
There is no reason for a module to require anope to be compiled with mysql support, strictly speaking, the module can check for the existance of the mysql dev libs (or any other libs a module may want) at compile time and link against them itsself should it find them. This removes any reliance on how anope is setup, and how anope access's its own db files.