This disables the following warnings:
C4100: unreferenced formal parameter
 (1035 occurrances)
C4214: nonstandard extension used : bit field types other than int
 (609 occurances)
C4996: 'avpriv_snprintf': This function or variable may be unsafe.
       Consider using _snprintf_s instead. To disable deprecation,
       use _CRT_SECURE_NO_WARNINGS. See online help for details.
 (351 occurrances)

Signed-off-by: Martin Storsjö <martin@martin.st>