Fails to build with CMake 4.0, outdated libvgm submodule

[OPNA2608]

The libvgm submodule needs an update:

cd libvgm && cmake -Bbuild -DBUILD_LIBAUDIO=OFF -DBUILD_LIBPLAYER=OFF -DBUILD_PLAYER=OFF -DBUILD_VGM2WAV=OFF -DUTIL_LOADERS=OFF
cc -Wall -O2 -MMD -c opm_file.c -o opm_file.o
CMake Error at CMakeLists.txt:2 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.


-- Configuring incomplete, errors occurred!
make: *** [Makefile:69: libvgm/build/bin/libvgm-emu.a] Error 1

…to at least ValleyBell/libvgm@4464b6d to fix the compatibility with CMake >= 4.0.

(an way to build against a system-installed libvgm would also be nice, to avoid issues with dependencies like this)