Disable SndIO by default on non-BSD systems

It can still be enabled by setting ALSOFT_BACKEND_SNDIO=TRUE with cmake, but
since it isn't loaded dynamically, don't create a requirement on libsndio when
it's not needed or wanted.

CMakeLists.txt

@@ -993,8 +993,12 @@ if(NOT WIN32)
         endif()
     endif()
 
-    # Check SndIO backend
-    option(ALSOFT_BACKEND_SNDIO "Enable SndIO backend" ON)
+    # Check SndIO backend (disabled by default on non-BSDs)
+    if(BSD)
+        option(ALSOFT_BACKEND_SNDIO "Enable SndIO backend" ON)
+    else()
+        option(ALSOFT_BACKEND_SNDIO "Enable SndIO backend" OFF)
+    endif()
     option(ALSOFT_REQUIRE_SNDIO "Require SndIO backend" OFF)
     if(ALSOFT_BACKEND_SNDIO)
         find_package(SoundIO)