build: plugins in WITH_PLUGINS should get mex_use_$name_plugin set to yes
build: plugins in WITH_PLUGINS should get mex_use_$name_plugin set to yes

WITH_PLUGINS is our way to define the default list of plugins enabled.
We need to ensure mex_use_$name_plugin is set to yes when the plugin is
getting enabled by being in $WITH_PLUGINS, not just when someone gives
--enable-$name on the configure command line.

media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer