Oh, I just remembered, after clicking configure in CMake for the 1st time, when the red options appear, you'll have to change certain settings, I unchecked everything except for enable-aufile (and BUILD_SHARED_LIBS of course), that's pretty much the bare minimum, which means you don't have anything to play the sound (since we have SFML for that), and enable-aufile allows fluidsynth to load MIDI files, which is all we need.
Trying using that same setting and see if it works.