Fixed in r359 (plcl) and r361 (me), it should now at least compile.
Don't know if it's working though, care to test it?
With ./configure --enable-ladspa, I am now (as of r361) getting these linker errors:
./.libs/libfluidsynth.so: undefined reference to `new_fluid_LADSPA_FxUnit'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_handle_start'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_handle_setnode'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_shutdown'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_handle_declnode'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_handle_clear'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_handle_add'
./.libs/libfluidsynth.so: undefined reference to `fluid_LADSPA_run'
collect2: ld returned 1 exit status
It seems like the file src/bindings/fluid_ladspa.c (which defines all of the above functions) is not being compiled or linked. I don't know enough about Autotools to fix this (having briefly tried).