Remove 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.

=== modified file 'configure.in'
--- configure.in	2009-11-24 12:26:08 +0000
+++ configure.in	2009-12-11 19:31:13 +0000
@@ -479,8 +479,6 @@
 fi ])
 
 dnl Some autoconf.h defines we might enable later...
-AC_DEFINE(USE_DISKIO_AIO, 0, [Whether POSIX AIO support is needed. Automatic])
-AC_DEFINE(USE_DISKIO_DISKTHREADS, 0, [Whether pthreads support is needed. Automatic])
 USE_AIOPS_WIN32=0
 use_aio=
 use_diskthreads=
@@ -646,6 +644,7 @@
 		;;
 	  esac
 	else
+	  AC_DEFINE(USE_DISKIO_AIO, 0, [Whether POSIX AIO support is needed. Automatic])
 	  AC_MSG_NOTICE([AIO DiskIO Module disabled. Missing POSIX AIO support.])
 	fi
 	;;