Prevent compiler warning on Windows if jmorecfg.h is included after the Windows headers, which also define FAR.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@891 632fc199-4ca6-4c93-a231-07263d6284db

jmorecfg.h

@@ -212,8 +212,11 @@ typedef unsigned int JDIMENSION;
  */
 
 #ifdef NEED_FAR_POINTERS
+#ifndef FAR
 #define FAR  far
+#endif
 #else
+#undef FAR
 #define FAR
 #endif