http://berklix.com/~jhs/src/bsd/fixes/FreeBSD/ports/gen/textproc/iso8879/Makefile.REL=8.1-RELEASE.diff
http://www.freebsd.org/cgi/query-pr.cgi?pr=146325

Put a hyphen before XMLCATMGR to ignore this error:

 cd /usr/ports/textproc/iso8879 ; make reinstall
  /usr/local/bin/xmlcatmgr -sc /usr/local/share/sgml/catalog.ports add CATALOG iso8879/catalog
  xmlcatmgr: entry already exists for `iso8879/catalog' of type `CATALOG'

# *** 8.1-RELEASE/ports/textproc/iso8879/Makefile	Sat Jul 17 18:00:56 2010
# --- new-generic/ports/textproc/iso8879/Makefile	Wed Jul 14 23:01:40 2010
# ***************
# *** 40,45 ****
#   	@${CHOWN} -R ${SHAREOWN}:${SHAREGRP} ${INSTDIR}
#   
#   post-install:
# ! 	@${XMLCATMGR} -sc ${CATALOG_PORTS_SGML} add CATALOG ${PORTNAME}/catalog
#   
#   .include <bsd.port.mk>
# --- 40,45 ----
#   	@${CHOWN} -R ${SHAREOWN}:${SHAREGRP} ${INSTDIR}
#   
#   post-install:
# ! 	@-${XMLCATMGR} -sc ${CATALOG_PORTS_SGML} add CATALOG ${PORTNAME}/catalog
#   
#   .include <bsd.port.mk>

8.1-RELEASE needs it.
9.2-RELEASE needs it.
Current 2013-11-06 needs it.
10.0-BETA2 needs it,  but with patch above it bleated:
	No such line 39 in input file, ignoring
	Hunk #1 succeeded at 31 with fuzz 1 (offset -9 lines).
   although it did apply, so to avoid a misleading "ignoring" from patch program,
   new patch below with changed line numbers.
10.1-RELEASE does not need it as no explicit command XMLCATMGR in label post-install:
11.0-CURRENT does not need it as no label post-install:

*** 10.0-BETA2/ports/textproc/iso8879/Makefile	Sat Oct 26 20:04:31 2013
--- new-generic/ports/textproc/iso8879/Makefile	Wed Nov  6 18:35:15 2013
***************
*** 31,36 ****
  	@${INSTALL_DATA} ${FILESDIR}/catalog ${INSTDIR}/catalog
  
  post-install:
! 	@${XMLCATMGR} -sc ${CATALOG_PORTS_SGML} add CATALOG ${PORTNAME}/catalog
  
  .include <bsd.port.mk>
--- 31,36 ----
  	@${INSTALL_DATA} ${FILESDIR}/catalog ${INSTDIR}/catalog
  
  post-install:
! 	@-${XMLCATMGR} -sc ${CATALOG_PORTS_SGML} add CATALOG ${PORTNAME}/catalog
  
  .include <bsd.port.mk>