docs: use rsvg-convert instead of inkscape (less dependencies), mention this in INSTALL

INSTALL

@@ -469,16 +469,19 @@ Building the documentation
 ==========================
 
 To build the documentation, Kwant has to be installed as described above.
-Additionally, the `sphinx documentation generator <http://sphinx.pocoo.org/>`_
-is required.
+Additionally, the `Sphinx documentation generator <http://sphinx.pocoo.org/>`_
+is required.  If PDF documentation is to be built, the tools from the `libRSVG
+<http://live.gnome.org/LibRsvg>`_ (Debian/Ubuntu package ``librsvg2-bin``) are
+needed to convert SVG drawings into the PDF format.
 
-HTML documentation can be built by entering the ``doc`` subdirectory of the
+HTML documentation is built by entering the ``doc`` subdirectory of the
 Kwant package and executing ``make html``.  PDF documentation is generated by
 executing ``make latex`` followed by a ``make all-pdf`` in ``doc/build/latex``.
 
-Because of some quirks of how sphinx works, it might be necessary to execute
+Because of some quirks of how Sphinx works, it might be necessary to execute
 ``make clean`` between building HTML and PDF documentation.  If this is not
-done, sphinx might mistakenly use PNG files for PDF output.
+done, Sphinx might mistakenly use PNG files for PDF output or other problems may
+appear.
 
 
 Hacking

doc/Makefile

@@ -120,7 +120,7 @@ doctest:
 	      "results in $(BUILDDIR)/doctest/output.txt."
 
 %.pdf: %.svg
-	inkscape --export-pdf=$@ $<
+	rsvg-convert -f pdf -o $@ $<
 
 # Make the image generation scripts by patching tutorial scipts.
 .SECONDARY: