EPUB Makefile target requires manually copying files around

[strk]

The dbtoepub tool doesn't seem to have a way to pass XSLT path so it doesn't work when images referenced via images/xxx are instead found in html/images/xxx

The debbie's script "solves" this by creating an images/ directory and copying all files there, but we need a more general fix

[strk]

On further analisys, it's not XSLT path that we are in need of, but rather images path, required at epub packaging time. Something to fix in here: ​https://github.com/docbook/xslt10-stylesheets/blob/fe16c90013b64e316c3e21ef92d1e8813c10f88c/xsl/epub/bin/lib/docbook.rb#L167

The code above just assumes image paths are relative to the path of the docbook input file (it'd be build dir)

[strk]

It's actually also XSLT path that we need, for postgis_special.ent I guess I could merge those entities back to postgis.xml to simplify life and be happy again with epub generation.

[Sandro Santilli <strk@…>]

In 52b6274d/git:

Make all images available via a single images/ directory in build dir

Symlinks static images into the same directory where generated
images are written.

Fixes the docs-install rule

Simplifies EPUB generation ( References #5422 )

[strk]

I filed upstream tickets:

• ​https://github.com/docbook/xslt10-stylesheets/issues/257
• ​https://github.com/docbook/xslt10-stylesheets/issues/258

[Sandro Santilli <strk@…>]

In 9245672/git:

Fix EPUB build (only in-tree builds)

Moves all postgis_special entities to top-level file,
symlink html images dir to make dbtoepub happy.

Simplify debbie epub generation code

Closes #5422

[robe]

debbie's builds appear to be broken now.

Giving error:

11:51:52 rm images # work around ​https://trac.osgeo.org/postgis/ticket/5422 11:51:52 rm: cannot remove 'images': No such file or directory

Also gitlab is broken for same reason

[Regina Obe <lr@…>]

In 3d725b6/git:

Add a -f to quiet warnings when folder doesn't exist
Closes #5422