Fix compilation on Mac OS X 10.4: Defining _POSIX_C_SOURCE hides the u_char &
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

Originally committed as revision 15306 to svn://svn.ffmpeg.org/ffmpeg/trunk
