However, for this particular issue, we need to tell java compiler, specified in build-impl.xml, where is(are) the "endorsed" folder(s), used by endorsed mechanism.
Note: "endorsed" folders is the list of folders, containing jar files that
overlap JDK classes.

Specifically, we need to specify javac compiler option:
-Djava.endorsed.dirs=absolute_path1:absolute_path2 (non shareable project)
or
-Djava.endorsed.dirs=${basedir}/lib/endorsed (shareable project)

So this is not about classpath but rather about folders.
Moreover, it would be good if project classpath could be extended with these endorsed jars (code completion, error badging,...)

и още:

The jaxws.endorsed.dir property in this file refers to files through absolute
classpath, which breaks sharability.
_ _ _

in private properties we specify some endorsed directories used for
compilation.
_ _ _

Then, we need to create a specific folder (like libs/endorsed) where all
endorsed jar files will be copied.

Всичко това е от http://netbeans.org/bugzilla/show_bug.cgi?format=multiple&id...

Явно endorsed е специфичен термин, който се отнася до други меща, не само и конкретно до (под)папки. Трябва да каже някой, дето е по-навътре в програмистките работи, аз съм дотук :(