I went to alfresco-community-sdk-4.2.b\src then extracted the alfresco-repository-src.zip
I wanted to build this part and create <strong>alfresco-repository-4.2.b.jar</strong> but can't find any build.xml anywhere. <strong>How do I build it?</strong>
You don't. The sdk is for you to build components and modules for alfresco. The source is provided for reference. The sdk is not intended to build alfresco itself.
If you want to build alfresco itself then you need to download from svn.