[PATCH] USE_DISKIO_ redefined

This patch removes the following ./configure warnings:

  conftest.cpp:34:1: warning: "USE_DISKIO_AIO" redefined
  conftest.cpp:35:1: warning: "USE_DISKIO_DISKTHREADS" redefined

The patch allows Squid v3.1 to build on RHEL 5.4 which has autoconf
v2.59. Without the patch, USE_DISKIO_AIO is zero but the corresponding
AIO files are compiled, leading to errors. I do not know if other
platforms are affected.

The patch is not required on my Ubuntu box but I did not investigate
why. It is possible that newer autoconf versions treat AC_DEFINE
redefinitions differently.

Thank you,

Alex.