Made libjpeg always use C linking so there are not incompatibilities with

other libjpeg header files.

dlib/external/libjpeg/jmorecfg.h

@@ -172,7 +172,11 @@ typedef unsigned int JDIMENSION;
 /* a function referenced thru EXTERNs: */
 #define GLOBAL(type)		type
 /* a reference to a GLOBAL function: */
+#ifdef __cplusplus
+#define EXTERN(type)		extern "C" type
+#else
 #define EXTERN(type)		extern type
+#endif
 
 
 /* This macro is used to declare a "method", that is, a function pointer.