Add logic for setting DRACO_ROUNDOFF_MODE for FMA CPP macro.

+ Add build system logic that allows us to toggle between *fast* and *accurate* FMA modes.  In
  general, *accurate* is required for CI and regression testing while *fast* is enabled for
  releases.
+ Related to #6
+ Added a new CMake option `DRACO_ROUNDOFF_MODE` that defaults to `ACCURATE`.
+ This option is published in `draco-config.cmake` and `ds++/config.h` so other projects are aware
  of Draco's run mode for FMA. The cmake file provide information to client build systems.  The
  header file provides information to client source code.
+ Release scripts were updated so that *fast* FMA is enabled for releases.