platform.h: better distinguishing between compilers.
platform.h: better distinguishing between compilers.

* Define OIIO_CLANG_VERSION and OIIO_APPLE_CLANG_VERSION.

* Change definition of OIIO_GNUC_VERSION to be 0 for clang, only have
  nonzero value for true gcc.

lgritz/oiio,lgritz/oiio,YangYangTL/oiio,YangYangTL/oiio,micler/oiio,lgritz/oiio,bdeluca/oiio,micler/oiio,OpenImageIO/oiio,YangYangTL/oiio,bdeluca/oiio,YangYangTL/oiio,bdeluca/oiio,OpenImageIO/oiio,lgritz/oiio,bdeluca/oiio,micler/oiio,micler/oiio,OpenImageIO/oiio,OpenImageIO/oiio