This was accomplished by adding some pretty nasty hacks to the FFmpeg recipe
so that we can compile it for the GCC2 ABI using GCC7. This works because
GCC's C ABI has not changed between GCC2 and GCC7.
As a consequence of this, pretty much all the longstanding issues of the
ancient-and-still-miscompiled FFmpeg 0.10 are now completely gone.
Fixes#5080, #8461, #12696, #12436, #13981#13410, #13337.
Closes (possibly fixed earlier) #8605, #8511, #6984.
Probably fixes (couldn't test) #13989, #11974.
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