autoconf: fix --disable-thread-support build under win32 Fixes: https://ci.appveyor.com/project/azat/libevent/build/job/gvud4tcqsd5bnarl Fixes: https://ci.appveyor.com/project/azat/libevent/build/2.1.6.80/job/5frnb1c3n4quxxqy Fixes: https://ci.appveyor.com/project/azat/libevent/build/2.1.6.80/job/3wdahbrew7setmoa