add waves audio backend when building on apple

libs/backends/wscript

@@ -1,12 +1,16 @@
 #!/usr/bin/env python
 from waflib.extras import autowaf as autowaf
 import os
+import sys
 
 # Mandatory variables
 top = '.'
 out = 'build'
 
-backends = [ 'jack', 'wavesaudio' ]
+backends = [ 'jack' ]
+
+if sys.platform == 'darwin':
+    backends += ['wavesaudio' ]
 
 def options(opt):
     autowaf.set_options(opt)