Rename README, LICENSE, BUILDING text files

The IJG README file has been renamed to README.ijg, in order to avoid
confusion (many people were assuming that that was our project's README
file and weren't reading README-turbo.txt) and to lay the groundwork for
markdown versions of the libjpeg-turbo README and build instructions.
diff --git a/jmemsys.h b/jmemsys.h
index 5026c7c..614ac7f 100644
--- a/jmemsys.h
+++ b/jmemsys.h
@@ -5,7 +5,8 @@
  * Copyright (C) 1992-1997, Thomas G. Lane.
  * It was modified by The libjpeg-turbo Project to include only code and
  * information relevant to libjpeg-turbo.
- * For conditions of distribution and use, see the accompanying README file.
+ * For conditions of distribution and use, see the accompanying README.ijg
+ * file.
  *
  * This include file defines the interface between the system-independent
  * and system-dependent portions of the JPEG memory manager.  No other