[Help-glpk] fix: make extern "C" automatic in header files

[Steven G. Johnson]

I'm using GLPK 4.8, and I notice that you still require C++ users to 
wrap extern "C" { ... } around the headers.

There is an easy way to make this automatic.  Just put:

#ifdef __cplusplus
extern "C" {
#endif    /* __cplusplus */

at the beginning of your header file and

#ifdef __cplusplus
}         /* extern "C" */
#endif    /* __cplusplus */

at the end.  (This is standard boilerplate that many C header files use 
in order to work with C++.)

I think that nesting extern "C" braces is allowed (g++ -pedantic -Wall 
allows it), so this shouldn't cause backwards-compatibility problems for 
existing C++ code that uses GLPK.

Cordially,
Steven G. Johnson