Bugfix: configure: Define default for use_libevent

Author: luke-jr

configure.ac

@@ -1506,6 +1506,7 @@ fi
 
 dnl libevent check
 
+use_libevent=no
 if test "$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench" != "nonononono"; then
   PKG_CHECK_MODULES([EVENT], [libevent >= 2.0.21], [use_libevent=yes], [AC_MSG_ERROR([libevent version 2.0.21 or greater not found.])])
   if test "$TARGET_OS" != "windows"; then