I'm writing this just in case somebody else is interested or in need of this. I just built a single header version of FluidSynth with NO glib dependency. It has only synth and soundfont capabilities (everything else was removed) and I haven't fully tested it yet, but it's working.
It is here:
This is all WIP.
PS.: I KNOW it is not pretty, but it WORKS and it is WAY easier to integrate.