[mythtv] Adding support for external ffmpeg-mt library

[Marc Tousignant]

I just tried to compile with your ffmpeg resync patch on 20788 and failed.

cpu_detect_x86_gcc.cpp: Assembler messages:
cpu_detect_x86_gcc.cpp:124: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:126: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:127: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:129: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:136: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:138: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:148: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:150: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:139: Error: bad register name `%rbx'
cpu_detect_x86_gcc.cpp:141: Error: bad register name `%rbx'
make[2]: *** [cpu_detect_x86_gcc.o] Error 1
make[2]: Leaving directory
`/var/tmp/portage/media-tv/mythtv-svn-9999/work/mythtv/libs/libmythsoundtouc
h'
make[1]: *** [sub-libmythsoundtouch-make_default] Error 2
make[1]: *** Waiting for unfinished jobs....
g++ -Wl,-O1 -Wl,--warn-common -Wl,--as-needed -Wl,-rpath-link,/libpostproc
-Wl,-rpath-link,/libswscale -Wl,-rpath-link,/libavfilter
-Wl,-rpath-link,/libavdevice -Wl,-rpath-link,/libavformat
-Wl,-rpath-link,/libavcodec -Wl,-rpath-link,/libavutil -Wl,-Bsymbolic
-Wl,-O1 -shared -Wl,-soname,libmythavutil-0.22.so.0 -o
libmythavutil-0.22.so.0.22.0 adler32.o aes.o avstring.o base64.o crc.o des.o
fifo.o integer.o intfloat_readwrite.o lfg.o lls.o log.o lzo.o mathematics.o
md5.o mem.o random_seed.o rational.o rc4.o sha1.o tree.o utils.o
-lpthread
/usr/lib/gcc/i686-pc-linux-gnu/4.3.2/../../../../i686-pc-linux-gnu/bin/ld:
warning: creating a DT_TEXTREL in object.
ln -s libmythavutil-0.22.so.0.22.0 libmythavutil-0.22.so
ln -s libmythavutil-0.22.so.0.22.0 libmythavutil-0.22.so.0
ln -s libmythavutil-0.22.so.0.22.0 libmythavutil-0.22.so.0.22
make[2]: Leaving directory
`/var/tmp/portage/media-tv/mythtv-svn-9999/work/mythtv/libs/libavutil'
make[1]: Leaving directory
`/var/tmp/portage/media-tv/mythtv-svn-9999/work/mythtv/libs'
make: *** [sub-libs-make_default-ordered] Error 2