Clean-up for alternate configurations.

* s/op_read_stereo_float/op_read_float_stereo/ for the fixed-point
   API.
* Fix compiler warnings exposed when optimizations are enabled.
* Fix opusfile_example to work with --enable-fixed-point
   --disable-float
* Fix seeking_example to not re-define OP_FIXED_POINT if it's
   already been defined.