Preventing --enable-plugins from disabling the plugins

The plugins are enabled by default, and the can be disabled by the
configure option --disable-plugins, but the option --enable-plugins
also led to disabling them.

configure.ac

@@ -125,7 +125,7 @@ AC_ARG_ENABLE(
 	[plugins],
 	[AS_HELP_STRING([--disable-plugins], [Disable external reparse point
 	 plugins for the ntfs-3g FUSE driver])],
-	,
+	[if test x${enableval} = "xyes"; then disable_plugins="no"; fi],
 	[disable_plugins="no"]
 )