The build plugin is part of the LWJGL Eclipse plugins. The following plugins are
required:

org.lwjgl
  Skeleton plugin, that is it contains the Eclipse Activator for setting up the
  native library path, but the lwjgl jars are missing. These jars are added by
  the build script later on

org.lwjgl.source
  Skeleton plugin for sources, plugin.xml etc are already configured, but again
  the sources itself are only added by the build script later on
  
org.lwjgl.doc
  Skeleton plugin for documentation, Javadoc is added by build script, however
  some Eclipse help files are already in place
  
org.lwjgl.info
  This plugin was called org.lwjgl.test in the elder version. I renamed it as it
  does not really contain a test but Eclipse views. Besides the spinning torus
  test view, I added an info view printing out current version of driver etc.
  and capabilities of user's OpenGL system
  
org.lwjgl.tools
  This plugin contains a library set up for plain Java projects, in order to get
  rid of defining additional parameters in the run configuration. It is
  documented in the help files. I have sent you this plugin once, but it never
  made it into the update site. Well, now it is in place
  
org.lwjgl.feature
  Feature plugin referencing the core lwjgl plugin. It contains two
  feature.xml definitions, one used for playing around in Eclipse, and a
  template which is used by the build script. If you ever change something
  here, pay attention to also change the template file.
  
org.lwjgl.feature.sdk
  Feature plugin referencing the src, doc, tools and info lwjgl plugin. 
  It contains two feature.xml definitions, one used for playing around in
  Eclipse, and a template which is used by the build script. If you ever change
  something here, pay attention to also change the template file.  
  
org.lwjgl.updatesite
  Well, this is the updatesite. As in the feature project, site.xml comes 
  together with a template version used by the build script.
  
org.lwjgl.build -- THIS Plugin
  This is the build script project. Although it is an Eclipse project, you can
  simply run the build script with ant from command line. See later on.

Everything is created in org.lwjgl.build/build. 
Here is how to build the update site:

1) Create a new folder in org.lwjgl.build/lwjgl-archives , use the version 
   number as folder name. E.g., for version 2.6 create a folder "2.6", 
   for version 2.6.1 "2.6.1" and so on.
2) Copy the lwjgl, lwjgl-docs and lwjgl-source zips into that folder. E.g., 
   for version 2.6, this folder must contain the following files: 
     org.lwjgl.build/lwjgl-archives
     	+ 2.6
     		- lwjgl-2.6.zip
     		- lwjgl-docs-2.6.zip
     		- lwjgl-source-2.6.zip
3) In order to run the build script, change to folder org.lwjgl.build and run
   ant with a parameter indicating the appropriate LWJGL version, e.g.
		ant -Dversion=2.6 dist

You will find the updatesite in
	org.lwjgl.build/build/plugins/org.lwjgl.updatesite

You may note that the jars are all signed. I have added a temporary keystore
with a dummy alias. In order to use your key, simply edit the properties
(line 11-14) in the build script. If you do not want to store the password in
the build script, simply define it via "-Dstorepass=****" in the command line.

Some additional remarks:
- The version number is automatically "normalized" to Eclipse standards, 
  that is 2.6 will become 2.6.0. You do not have to rename the initial archives,
  you can use 2.6. Or 2.6.1 ;-)
- The lwjgl-debug.jar is not added to the org.lwjgl plugin, as I do not know
  how to select one or the other. But I have not further investigated in 
  that direction.
- Note that only the created plugin and feature jars are signed, and not the
  nested lwjgl.jar etc., as I figure that you will sign these jars yourself
  when creating the zip files.
- When testing the update site and the installation of plugins, pay attention
  to the Eclipse P2 system, which caches a lot of things. Simply uninstalling
  a plugin does not remove the plugins from the Eclipse installation. When
  the very same version is re-installed, instead of loading new files from the
  update site these pre-installed versions are reused!