devel/doxygen: Fix parallel examples build (under DOCS)

Tatsuki Makino wrote the original patch vs. Doxygen 1.9.1, but it no
longer applied, so I manually re-did following his example.

While here, ditch the :noninja, 1.9.6 builds fine with Ninja, too.

Obtained from: tatsuki_makino@hotmail.com
PR: 259331
Approved by: maintainer timeout (fluffy@, 39 months)