easy fix: append-flags -D__STDC_CONSTANT_MACROS (UINT64_C isnt standard in C++ unless this macro is defined, and ffmpeg headers require it)
Created attachment 235629 [details] Build log (compressed)
Thanks, I've applied the workaround to the 5.0.x.x versions. Older ebuilds already use it. *** This bug has been marked as a duplicate of bug 318969 ***