TurboJPEG: Make global error handling thread-safe

... on platforms that support thread-local storage.  This currently
includes all supported platforms except 32-bit macOS.

Fixes #396

turbojpeg.h

@@ -1650,7 +1650,7 @@ DLLEXPORT void tjFree(unsigned char *buffer);
  * @param handle a handle to a TurboJPEG compressor, decompressor, or
  * transformer instance, or NULL if the error was generated by a global
  * function (but note that retrieving the error message for a global function
- * is not thread-safe.)
+ * is thread-safe only on platforms that support thread-local storage.)
  *
  * @return a descriptive error message explaining why the last command failed.
  */