After installing port textproc/docbook it is not possible to install textproc/docbook-xsl.

The latter depends on the former, but the "install" target does not recognize its presence. It tries to "make install", and this fails because the port is already installed.

In other words:
 - textproc/docbook-xsl depends on textproc/docbook. This means that, for instance, portupgrade installs textproc/docbook before textproc/docbook-xsl.
 - textproc/docbook-xsl's "install" target does not recognize the presence of the installed textproc/docbook. Therefore it tries to run "make install" on textproc/docbook; this fails, because textproc/docbook is already installed.

Fix: 

This is not a fix, but a workaround:
1- delete installed package docbook-1.4
2- run "make install" for port textproc/docbook-xsl

In other words, docbook-xsl will install if it is installed _without_ docbook.
How-To-Repeat: diavoletto# portupgrade -N docbook-xsl
[Gathering depends for textproc/docbook-xsl . done]
--->  Installing 'docbook-1.4' from a port (textproc/docbook)
--->  Building '/usr/ports/textproc/docbook'
===>  Cleaning for docbook-1.4
===>  Vulnerability check disabled, database not found
===>  Found saved configuration for docbook-1.4
===>  Extracting for docbook-1.4
===>  Patching for docbook-1.4
===>  Configuring for docbook-1.4
--->  Installing the new version via the port
===>  Installing for docbook-1.4
===>   Generating temporary packing list
===>  Checking if textproc/docbook already installed
===>   Registering installation for docbook-1.4
===>  Cleaning for docbook-1.4
[Updating the pkgdb <format:bdb_btree> in /var/db/pkg ... - 150 packages found (-0 +1) . done]
--->  Installing 'docbook-xsl-1.75.1' from a port (textproc/docbook-xsl)
--->  Building '/usr/ports/textproc/docbook-xsl'
===>  Cleaning for docbook-xsl-1.75.1
===>  Vulnerability check disabled, database not found
===>  Found saved configuration for docbook-xsl-1.74.0_1
===>  Extracting for docbook-xsl-1.75.1
=> MD5 Checksum OK for docbook-xsl-1.75.1.tar.bz2.
=> SHA256 Checksum OK for docbook-xsl-1.75.1.tar.bz2.
=> MD5 Checksum OK for docbook-xsl-doc-1.75.1.tar.bz2.
=> SHA256 Checksum OK for docbook-xsl-doc-1.75.1.tar.bz2.
/usr/bin/find /usr/ports/textproc/docbook-xsl/work/docbook-xsl-1.75.1 -name '.#*' -or -name '.gitignore' -delete
/usr/bin/find /usr/ports/textproc/docbook-xsl/work/docbook-xsl-1.75.1/extensions -type d -mindepth 1 -maxdepth 1 -print0 | /usr/bin/xargs -0 /bin/rm -rf
===>  Patching for docbook-xsl-1.75.1
===>  Configuring for docbook-xsl-1.75.1
--->  Installing the new version via the port
===>  Installing for docbook-xsl-1.75.1
===>   docbook-xsl-1.75.1 depends on file: /usr/local/share/xml/docbook - not found
===>    Verifying reinstall for /usr/local/share/xml/docbook in /usr/ports/textproc/docbook
===>  Vulnerability check disabled, database not found
===>  Found saved configuration for docbook-1.4
===>  Extracting for docbook-1.4
===>  Patching for docbook-1.4
===>  Configuring for docbook-1.4
===>  Installing for docbook-1.4
===>   Generating temporary packing list
===>  Checking if textproc/docbook already installed
===>   docbook-1.4 is already installed
      You may wish to ``make deinstall'' and install this port again
      by ``make reinstall'' to upgrade it properly.
      If you really wish to overwrite the old port of textproc/docbook
      without deleting it first, set the variable "FORCE_PKG_REGISTER"
      in your environment or the "make install" command line.
*** Error code 1

Stop in /usr/ports/textproc/docbook.
*** Error code 1

Stop in /usr/ports/textproc/docbook.
*** Error code 1

Stop in /usr/ports/textproc/docbook.
*** Error code 1

Stop in /usr/ports/textproc/docbook-xsl.
*** Error code 1

Stop in /usr/ports/textproc/docbook-xsl.
*** Error code 1

Stop in /usr/ports/textproc/docbook-xsl.
===>  Cleaning for docbook-1.4
===>  Cleaning for docbook-xsl-1.75.1
diavoletto#