fluidsynth 2.1 RC1

Release candidate of fluidsynth 2.1.0 as it is planned to be released in late November. For more details refer to the mailing list:

https://lists.nongnu.org/archive/html/fluid-dev/2019-10/msg00012.html

Prebuilt binaries for Android (#574, thanks to @JonLatane) and WindowsXP (and later) are attached and for the future will be available as CI artifacts.