FFmpeg is written in C99, thus some features may not be compilable or usable in C++.

Anyway, for most purposes, including FFmpeg headers in a C++ application should be rather straightforward.

First, to include the FFmpeg headers within your C++ application you need to explicitly state that you are including C code. You can do this by encompassing your FFmpeg includes using {{{extern "C"}}}, like in:
{{{
extern "C" {
#include <libavutil/imgutils.h>
#include <libavcodec/avcodec.h>
#include <libswscale/swscale.h>
}
}}}

Second, you may need to append {{{-D__STDC_CONSTANT_MACROS}}} to your CXXFLAGS flags, if the compiler complains about ’UINT64_C’ was not declared in this scope.