Fix stk/rawwaves for msys2

[tresf]

Enables Mallets on MSYS2

• Fixes the calculation of the rawwaves search path in msys2.
• Makes STK_RAWWAVE_ROOT mandatory (as it should have always been)

  Optional plugins
  ----------------
  * Lv2 plugins                 : OK
  * SUIL for plugin UIs         : OK
  * ZynAddSubFX instrument      : OK
  * Carla Patchbay & Rack       : OK (weak linking enabled)
  * SoundFont2 player           : OK
  * Sid instrument              : OK
- * Stk Mallets                 : not found, please install libstk0-dev (or similar) 
+ * Stk Mallets                 : OK
  [...]

Depends:

• [new-package] stk 5.0.1 msys2/MINGW-packages#23541

TODO:

• Add pacboy -S stk:b to the MSYS2 wiki

[messmerd]

LGTM

[tresf]

LGTM

Still waiting on upstream, so I'll give it a bit. Commit history shows the maintainers are very active over there, so hopefully soon.