If jpeglib.h is included in a C++ program, the functions in
	libjpeg cannot be linked against, because the function prototypes
	are not decorated with 'extern "C"'

Fix: The fix is to re define the EXTERN macro if you are compiling with
a C++ compiler.

How-To-Repeat: If you put the following code snippet:
#include <stdio.h>
#include "jpeglib.h"
int main(int argc, char *argv[]) { jpeg_std_error(NULL); }

in a C file (test.c), and compile with:
gcc -I/usr/local/include test.c -L/usr/local/lib -ljpeg

it will compile and link fine.

If you rename the file to test.cpp, and compile with:
g++ -I/usr/local/include test.cpp -L/usr/local/lib -ljpeg

You will get a link error:
: undefined reference to `jpeg_std_error(jpeg_error_mgr*)'