I'm not sure how you are currently obtaining FluidSynth, which would probably determine where to start.
Here is a likely semi-outdated general guide on building FluidSynth:
If you are using the autconf ./configure script, then you would pass "--enable-ladspa" switch to the configure script, like so:
The cmake build system is easier in my opinion, but again, I'm not sure how you are building for your platform, so it may or may not be easier. With cmake, you just enable LADSPA by enabling the enable-ladspa option. CMake has several front ends. I use the "ccmake" interactive curses terminal interface. Basically:
c # To configure FluidSynth - install development libraries as needed and keep pressing "c" after installing each one, until it succeeds
Find the enable-ladspa option and switch it to ON
c # Hit 'c' again to configure after changing options
g # g generates the build files
q # Quit ccmake
make # Build FluidSynth
Maybe that helps?