Find docbook-xsl and dblatex templates on Red Hat/Fedora

Find docbook-xsl and dblatex templates on Red Hat/Fedora

`/usr/share/sgml/docbook/xsl-stylesheets` and `/usr/share/dblatex` are
places where docbook-style-xsl and, respectively, dblatex packages on
Red Hat systems put their XSL templates. Unless we hint this place it
has to be added to `./configure` manually (`--with-docbook-xsl=...`):
https://src.fedoraproject.org/rpms/bind/blob/master/f/bind.spec#_691.

On Fedora 30:

Before
```
./configure
...
checking for Docbook-XSL path... auto
checking for html/docbook.xsl... "not found"
checking for xhtml/docbook.xsl... "not found"
checking for manpages/docbook.xsl... "not found"
checking for html/chunk.xsl... "not found"
checking for xhtml/chunk.xsl... "not found"
checking for html/chunktoc.xsl... "not found"
checking for xhtml/chunktoc.xsl... "not found"
checking for html/maketoc.xsl... "not found"
checking for xhtml/maketoc.xsl... "not found"
checking for xsl/docbook.xsl... "not found"
checking for xsl/latex_book_fast.xsl... "not found"
```

After:
```
./configure
...
checking for Docbook-XSL path... auto
checking for html/docbook.xsl... /usr/share/sgml/docbook/xsl-stylesheets/html/docbook.xsl
checking for xhtml/docbook.xsl... /usr/share/sgml/docbook/xsl-stylesheets/xhtml/docbook.xsl
checking for manpages/docbook.xsl... /usr/share/sgml/docbook/xsl-stylesheets/manpages/docbook.xsl
checking for html/chunk.xsl... /usr/share/sgml/docbook/xsl-stylesheets/html/chunk.xsl
checking for xhtml/chunk.xsl... /usr/share/sgml/docbook/xsl-stylesheets/xhtml/chunk.xsl
checking for html/chunktoc.xsl... /usr/share/sgml/docbook/xsl-stylesheets/html/chunktoc.xsl
checking for xhtml/chunktoc.xsl... /usr/share/sgml/docbook/xsl-stylesheets/xhtml/chunktoc.xsl
checking for html/maketoc.xsl... /usr/share/sgml/docbook/xsl-stylesheets/html/maketoc.xsl
checking for xhtml/maketoc.xsl... /usr/share/sgml/docbook/xsl-stylesheets/xhtml/maketoc.xsl
checking for xsl/docbook.xsl... /usr/share/dblatex/xsl/docbook.xsl
checking for xsl/latex_book_fast.xsl... /usr/share/dblatex/xsl/latex_book_fast.xsl
```