The default value of the myisam_max_extra_sort_file_size could be
higher than the maximum accepted value, leading to warnings upon
the server start.

The solution is to simply set the value to the maximum value in a
32-bit built (2147483647, one less than the current). This should
be harmless as the option is currently unused in 5.1.

include/myisam.h:
  Remove now-unused macro.
sql/mysqld.cc:
  Set max value to INT_MAX32.