build: Fix maintainer-mode and embedded FFmpeg configuration parameters
Don't use AC_CONFIG_SUBDIRS to call the FFmpeg configure script, as it complains about all the unknown parameters autoconf gives it, and fiddling with ac_configure_args makes maintainer-mode call our real configure script with a bunch of bogus arguments. Instead, use AC_CONFIG_COMMANDS to call the FFmpeg configure script ourselves. Remove autogen.sh code that modifies the FFmpeg configure script, as it's not needed now that we only pass it arguments it understands, and move the detection of flags like --disable-ffmpeg into the configure script, otherwise they never get passed to FFmpeg if we call configure ourselves, such as from a tarball.
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