Mingw_w64 not linking FFmpeg module (Bug #984)

Not clear whether this is a problem with the build system or the included 3rdparty FFmpeg 64-bit libs, but the linker throws many missing symbol av_*** errors. Not sure what's going wrong as the generated makefiles appear to link against the appropriate libs and the libs appear to have the appropriate symbols.

Using sezero's latest Mingw_w64 toolchain (and was successfully able to build Qt 4.7 git master branch with it). Ran into the same error in both the trunk (rev. 4857) as well as the official 2.2 release. Happy to provide more info if needed though I suspect it should be pretty easy to replicate.

In general, not having 64-bit windows video support per tickets (#738, #677) is a serious deficiency. Hope you will consider making this a priority for 2.3 release. Many thanks in advance!