Back out the weak linkage to libavformat

This causes build issues on both Windows and OS X.  It's not worth a slightly
"easier" build on Linux if it breaks our other two platforms completely.

Patch from #9468

mythtv/libs/libmyth/libmyth.pro

@@ -87,6 +87,7 @@ LIBS += -L../libmythfreesurround -lmythfreesurround-$${LIBVERSION}
 LIBS += -L../../external/FFmpeg/libavcodec -lmythavcodec
 LIBS += -L../../external/FFmpeg/libavcore  -lmythavcodec
 LIBS += -L../../external/FFmpeg/libavutil  -lmythavutil
+LIBS += -L../../external/FFmpeg/libavformat  -lmythavformat
 
 TARGETDEPS += ../libmythsamplerate/libmythsamplerate-$${MYTH_LIB_EXT}
 TARGETDEPS += ../libmythsoundtouch/libmythsoundtouch-$${MYTH_LIB_EXT}

mythtv/libs/libmyth/spdifencoder.h

@@ -11,18 +11,6 @@ extern "C" {
 #include "libavcodec/audioconvert.h"
 }
 
-#pragma weak av_register_all
-#pragma weak av_guess_format
-#pragma weak avformat_alloc_context
-#pragma weak av_new_stream
-#pragma weak av_write_header
-#pragma weak av_write_frame
-#pragma weak av_write_trailer
-#pragma weak avcodec_close
-#pragma weak av_freep
-#pragma weak av_set_parameters
-#pragma weak av_alloc_put_byte
-
 class MPUBLIC SPDIFEncoder
 {
   public: