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 235625 [details] Build log
+ 21 Jun 2010; Samuli Suominen <ssuominen@gentoo.org> gnash-0.8.7.ebuild: + Fix building with FFmpeg >= 0.6 wrt #324357 by Diego E. Pettenò.