The Porter's Handbook expands the variables DATADIR, DOCSDIR and
EXAMPLESDIR as PREFIX/shared/PORTNAME, PREFIX/shared/doc/PORTNAME and
PREFIX/shared/examples/PORTNAME respectively.
As it can be checked by running "make -V" in any port directory, they
are expanded into PREFIX/shared/PORTNAME, PREFIX/shared/doc/PORTNAME and
PREFIX/shared/examples/PORTNAME instead.