Unable to load Soundfonts with MinGW 8.1

[derselbst]

FluidSynth version

<=2.5.2

Describe the bug

When compiling fluidsynth with MinGW 8.1, fluidsynth will be unable to load SoundFonts. (Possibly also MIDIs and DLS). That's because when fluidsynth seeks in a file using SEEK_CUR, _fseeki64 is used, which in turn calls _ftelli64 to seek from the current offset. However, _ftelli64 reports incorrect values in this toolchain, see: https://sourceforge.net/p/mingw-w64/bugs/864/

This causes fluidsynth to seek to the wrong offset, read wrong data, and ultimately fail.

Expected behavior

SEEK_CUR uses the correct file offset, as reported by ftell() or fgetpos().

Steps to reproduce

See unit test and MinGW8.1 CI build in linked PR.

Additional context

No response