gdcm2 @2.8.9: xsltParseStylesheetProcess : document is not a stylesheet

[ryandesign (Ryan Carsten Schmidt)]

gdcm2 is failing to build everywhere:

​https://build.macports.org/builders/ports-13_arm64-builder/builds/33722/steps/install-port/logs/stdio

error : Unknown IO error
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl-ns/current/manpages/docbook.xsl"
error
xsltParseStylesheetFile : cannot parse http://docbook.sourceforge.net/release/xsl-ns/current/manpages/docbook.xsl
compilation error: file /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_science_gdcm2/gdcm2/work/build/Utilities/doxygen/gdcmxml.xml line 4 element refentry
xsltParseStylesheetProcess : document is not a stylesheet
make[2]: *** [Utilities/doxygen/gdcmxml.1] Error 5
make[2]: Leaving directory `/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_science_gdcm2/gdcm2/work/build'
make[1]: *** [Utilities/doxygen/CMakeFiles/DOCBOOK_MANPAGES.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

This is weird because gdcm2 does declare a dependency on docbook-xsl-ns which should provide that.

This was previously reported in #61140 as a result of which that dependency was added.