MYSQL_ADD_PLUGIN fixes

* use DISABLED for real (was ignored)
* add ONLY_IF (conditional disabled)
* remove BUILD_ prefix, now it's only PLUGIN_XXX=NO|STATIC|DYNAMIC|AUTO
* add PLUGIN_XXX=FORCE
* make sure all plugins always do MYSQL_ADD_PLUGIN for FORCE to work
* default PLUGIN_XXX value when WITH_XXX is set is now STATIC when plugin
  supports it and AUTO otherwise