PDF manual cannot be built with out-of-tree builds

[strk]

Wrong path for the custom style:

if test x"/usr/bin/dblatex" = x; then \
   echo "Error: dblatex not found, can't build pdf"; \
   echo "       try installing dblatex and then re-run configure"; \
   false; \
else \
        /usr/bin/dblatex -T native -t pdf \
           -I "/usr/local/src/postgis/postgis/build/pg13/doc/html" \
           -P doc.collab.show=0 \
           -P figure.note="/usr/local/src/postgis/postgis/build/pg13/doc/html/images/note" \
           -P figure.tip="/usr/local/src/postgis/postgis/build/pg13/doc/html/images/tip" \
           -P figure.important="/usr/local/src/postgis/postgis/build/pg13/doc/html/images/important" \
           -P figure.warning="/usr/local/src/postgis/postgis/build/pg13/doc/html/images/warning" \
           -P figure.caution="/usr/local/src/postgis/postgis/build/pg13/doc/html/images/caution" \
           -P latex.output.revhistory=0 \
           -s "/usr/local/src/postgis/postgis/build/pg13/doc/texstyle.sty" \
           -o postgis-3.4.0dev-ru.pdf postgis-out.xml; \
fi
Error: Latex style '/usr/local/src/postgis/postgis/build/pg13/doc/texstyle.sty' not found

Evidently none of our bots are building the PDFs, this is something to fix too

[Sandro Santilli <strk@…>]

In 2e6f30da/git:

Fix out-of-tree pdf building (finding texstyle.sty)

References #5413

[strk]

I didn't close this ticket yet because I still have problems with XSLT paths (dblatex searches for postgis_special.xml under ent dir for some reason)

[Sandro Santilli <strk@…>]

In d9f1ec0/git:

Use absolute paths for XSLT, stop using subdir for entity file

Fixes #5416
Fixes #5413

[strk]

Reopening once again, as out-of-tree build still fails due not finding images, like: Image 'images/PostGIS_logo.png' not found

[Sandro Santilli <strk@…>]

In b467355d/git:

Fix out-of-tree PDF build by adding srcdir to images path

Closes #5413