ffmpeg: remove unused variable

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
dsputil: make selectable.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/os_support: use 64-bit fstat/lseek variants for MSVC as well

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>