That page was useful, but it is a few years out of date and doesn't cater for the latest Android NDKs.
Can you try:
https://bitbucket.org/phil_blandford/androidfluidsynth/src/master/
This has a script to build glib and its dependencies for Android (actually glib was pretty easy, it was the dependencies that needed more work), assuming you have a Debian-based build machine.
I don't think the issue of an Android glib build should be insuperable.
There does need to be a user-space driver that writes to the native Android sound layer - I tried to get the one by Astushi Eno working, with only partial success.
Any future one I think should use the new Oboe API, which is extremely simple to use, has automatic latency correction, and chooses between OpenSLES and AAudio depending on Android version.
HOWEVER, there is an Android Synth library called SherlockMidi, which I am using in my current project - I've found it to work nearly flawlessly across every device I've tried (occasionally there's a burst of distortion on app startup on some devices, that's the only problem I've found) - for Soundfont support and note on/off events, it may be better to use that, since Fluidsynth on Android is currently looking a long, long way off.
Better still, it's Apache 2.0 license.