libevent: fix sslSupport = false

pkgs/development/libraries/libevent/default.nix

@@ -20,6 +20,8 @@ stdenv.mkDerivation rec {
     })
   ];
 
+  configureFlags = lib.optional (!sslSupport) "--disable-openssl";
+
   preConfigure = lib.optionalString (lib.versionAtLeast stdenv.hostPlatform.darwinMinVersion "11") ''
     MACOSX_DEPLOYMENT_TARGET=10.16
   '';