Unconditionally define memcmp() if not sane

Prior to this commit, if there was a #define for memcmp that invoked a
version that Configure deemed to not be sufficient for normal use, it
was retained, so that perl used the defective version.  This apparently
hasn't been a problem in the field, but I realized the potential issue
doing code reading, and am correcting it.

perl.h

@@ -1041,9 +1041,8 @@ EXTERN_C int usleep(unsigned int);
 #    endif
 #  endif
 #else
-#   ifndef memcmp
-#	define memcmp 	my_memcmp
-#   endif
+#   undef memcmp
+#   define memcmp   my_memcmp
 #endif /* HAS_MEMCMP && HAS_SANE_MEMCMP */
 
 #ifndef memzero