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

libavcodec/ppc/check_altivec.c

@@ -23,6 +23,7 @@
  */
 
 #ifdef __APPLE__
+#undef _POSIX_C_SOURCE
 #include <sys/sysctl.h>
 #elif __AMIGAOS4__
 #include <exec/exec.h>