wildmidi.c: use WildMidi_GetVersion(). formatting cleanups.
reverb.c: replace sqrtf() usage by sqrt().
wildmidi_lib.h (WM_SYMBOL): apply visibility attributes only during library build
CMAKE:  make WILDMIDI_CFG a configurable option. add OPENAL_INCLUDE_DIR
only to player and if AUDIODRV_OPENAL is set. always add WILDMIDI_BUILD
definition to library builds, not just when targetting windows.