Problems building Arduino source.

I am building Arduino IDE from Arduino source code, following the official steps at:

I have followed each step carefully, in fact everything seems well, but I get an error at line 35 in build.xml, this is:

This is what I get after apply “ant”:

C:\arduino\build\build.xml:35: The following error occurred while executing this line:
C:\arduino\build\build.xml:499: taskdef class net.sf.launch4j.ant.Launch4jTask cannot be found
using the classloader AntClassLoader

I don’t know what to do in order to go on, can anyone help me? By the way, I use windows 7 64 bits.