[fluid-dev] let cmake find WinMidi and DSound

[Carlo Bramini]

Hello,

I was thinking on making a fix for moving DSOUND_SUPPORT and WINMIDI_SUPPORT 
from fluidsynth_priv.h to config.h, but hopefully I have seen that a branch 
already exists with this fix:

https://github.com/FluidSynth/fluidsynth/commit/74ea46c2a9d4d4a0e6fb1c592cd602f044505768

Actually, this is more or less what I also wanted to do, so I was wondering if 
there is a particular reason because it has never been applied to the master.

Sincerely.