Jetty BUILD FAILURE on java 1.4

Compiling 43 source files to /src/jetty-6.1.0/modules/util/target/classes
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Compilation failure

/src/jetty-6.1.0/modules/util/src/main/java/org/mortbay/util/ajax/AjaxFilter.java:[22,-1] cannot access javax.servlet.Filter
bad class file: /.m2/repository/javax/servlet/servlet-api/2.5/servlet-api-2.5.jar(javax/servlet/Filter.class)
class file has wrong version 49.0, should be 48.0

So servlet-api-2.4.jar compiled with J2SE 1.5 and according to the
"
Changes from Maintenance Review 3, accepted on 27 September 2005"

...

6. Minimum J2SE Requirement

Servlet 2.5 Containers now require J2SE 5.0 as the minimum Java version. SRV.1.2 "What is a Servlet Container?" was updated to reflect this requirement."

Seems like ok.
But than the question: how could we compile jetty with 1.4
REASME says:
========================================
BUILDING JETTY
==============

Jetty uses maven 2 as its build system. Maven will fetch
the dependancies, build the server and assemble a runnable
version:

mvn install

Jetty itself only needs java 1.4, however to build JSP 2.1
support you need to use java5 AND you will need to have
cvs installed. If you want to use java1.4,
then you can use the jsp-2.0 modules instead of the
jsp-api-2.1 and jsp-2.1 modules.
============================================

Looks straightforward, but...