Use extern "C" in headers

[jgcodes2020]

All FFmpeg headers should begin and end as follows:

// _GUARD_MACRO_ should be replaced with the header's specific guard macro
#ifndef _GUARD_MACRO_
#define _GUARD_MACRO_
#ifdef __cplusplus
extern "C" {
#endif

// header code goes here

#ifdef __cplusplus
}
#endif
#endif

The extern "C" block ensures that FFmpeg's symbols are treated as C symbols in C++, thus removing the need to manually place them in an extern "C" block.

Since this ticket applies to more than one component it has been left as undetermined.

[Cigaes]

FFmpeg is not a c++ program, we make no effort to make the headers compatible, we will not pretend we do. If you want to use FFmpeg in a c++ project, include the headers as you see fit, and maybe you will be lucky.

[Balling]

C and C++ cannot be made compatible by just extern alone! This is nuts.