Fix potential crashes in case fluid-synth runs into an OOM error, and address a const-cast compiler warning. Switch to track github repo (instead of sf.net git)
This is a stripped down version of fluidsynth (library only) from git://git.code.sf.net/p/fluidsynth/code-git revisition f52597be038a5a045fc74b6f96d5f9b0bbbbc044 from May/2015 imported into Ardour Aug/2016 fluidsynth is licensed in terms of the LGPL-2+, see individual source files for (C) holders.