[mythtv] Build system issues with FFmpeg 5.1

[Scott Theisen]

Hello all,

When I copy https://github.com/ulmus-scott/FFmpeg/tree/release5.1 into 
MythTV and constify some pointers in libmyth/audio to satisfy 
-fpermissive, I get the following error:

```
make[2]: *** No rule to make target 
'../../external/FFmpeg/libswresample/libmythswresample.so.', needed by 
'libmyth-33.so.33.0.0'.  Stop.
```

It should be `libmythswresample.so.4`.  In 
`mythtv/libs/libmyth/Makefile`, libmythavutil.so.57 has its version 
number, but libmythavcodec also is missing its version number.

I suspect this is due to FFmpeg splitting their version macros across 
two files, version_major.h and version.h.  libavutil has an empty 
version_major.h, only using version.h.

The problem appears to be in configure in get_version() on line 7749, 
but I have no idea how to fix it.  (Concatenating both files before 
processing with awk?)

Any suggestions are greatly appreciated.

Thanks,

Scott