The libav implementation has been removed from ffmpeg as it clashes with
existing implementation.

Since yasm emms is not required for 32b but is required for 64b we
include it all the time so that we can support both configurations in a
single project.

Debug can be selected in the output VS projects and cannot be set via
command line.

Also made sure that all enabled by default options are auto enabled
which removes the need for certain --enable-xxx options.

Debug builds wont compile without Dead Code Elimination this requires O2
optimization. The Oy- to re-enable frame pointers and default runtime
checks to avoid overriding optimization settings.

Update VS project generator to support recent merge changes (fix for multiple config options with different prefix)