CMake: Default HAVE_AUDIO on.

dtool/Config.cmake

@@ -788,6 +788,9 @@ option(HAVE_EGG
 avoid building this, unless you really want to make a low-footprint
 build (such as, for instance, for the iPhone)." ON)
 
+option(HAVE_AUDIO
+  "Do you want to build the audio interface?" ON)
+
 # These image formats don't require the assistance of a third-party
 # library to read and write, so there's normally no reason to disable
 # them int he build, unless you are looking to reduce the memory footprint.