Commons Lang uses Maven or Ant as a build system.
You may also be interested in the upgrade notes:
Upgrade from 2.2 to 2.3
Upgrade from 2.1 to 2.2
Upgrade from 2.0 to 2.1
Upgrade from 1.0 to 2.0
To build a jar file, change into the root directory of Lang and run "maven jar". The result will be in the "target" subdirectory.
To build the Javadocs, run "maven javadoc". The result will be in "target/docs/apidocs".
To build the full website, run "maven site". The result will be in "target/docs". You must be online to successfully complete this target.
Further details can be found in the commons build instructions.