Right. I noticed in the wiki RSP is not a build option: http://wiki.fireflymediaserver.org/Configure_Options
so I checked my own admin page, and saw RSP is under plugins.
maybe you should enable the plugin or point to the correct plugin dir?
Your currently running plugins are shown on the home of the adminpage of firefly.
Perhaps you can copy the plugin from the precompiled package, and add it to your own compiled version which has flac enabled.