Be more explicit about SPIRV_WERROR option. (#2141)

There was some confusion about the SPIRV_WERROR option when it is turned
off.  When the option is off nothing is done.  That is, we do not add
-Wno-error.  This means that if the parent project added -Werorr to the
C flags or CXX flags, then warnings will still be treated as errors.

I've updated the README.md to make this explicit.

Fixes #2121.