configure: Add neat summary of settings, from lldpd

Hat tip to @vincentbernat

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

m4/plugin.m4

@@ -30,6 +30,7 @@ Optional Plugins:])
     fi])
     if test "x$enable_plugin" = "xyes"; then
        	    AC_DEFINE([HAVE_]PLUGGY[_PLUGIN], 1, [Define to 1 if the $1 plugin is enabled.])
+	    plugins="$1 $plugins"
     fi
     AM_CONDITIONAL([BUILD_]PLUGGY[_PLUGIN], test "x$enable_plugin" = "xyes")
     enable_pluggy="$enable_plugin"])