Make it buildable with -ansi flag on some platforms.

extra/innochecksum.c:
  Define _XOPEN_SOURCE in order to include getopt.h when needed.
  
  The problem is that some platforms (for instance,
  SuSE 9.1/gcc-3.3.3) do not include getopt.h unless _XOPEN_SOURCE
  is defined, so optarg is undefined and the compiler complains.

extra/innochecksum.c

@@ -28,6 +28,8 @@
 #define _LARGEFILE_SOURCE
 #define _LARGEFILE64_SOURCE
 
+#define _XOPEN_SOURCE /* needed to include getopt.h on some platforms. */
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>