similar typedefs that sysctl.h needs. Since sysctl() itself isn't POSIX
undefining _POSIX_C_SOURCE for check_altivec.c seems the best way to fix this.
patch by David Conrad lessen42 at gmail com 


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15306 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b