Don't use google fonts when building html doc.

  Asciidoctor uses google fonts in its html documentation, which
  makes the html files not fully usable offline. This triggered
  a warning in lintian which would prefer all docs are fully ofline
  and don't download stuff automatically.

  This disables the webfonts from the html generation, so the html
  files are viewed the same offline and no longer automatically download
  google's webfonts.

doc/Makefile.am

@@ -58,6 +58,6 @@ $(html_MANS): $(SECTION_FILES)
 	"$(ASCIIDOC)" -b manpage -a "$(patsubst %.1,%,$@)" "$<" -o "$@"
 
 %.html: %.adoc
-	"$(ASCIIDOC)" -b html5 -a toc -a "$(patsubst %.html,%,$@)" "$<" -o "$@"
+	"$(ASCIIDOC)" -b html5 -a toc -a webfonts! -a "$(patsubst %.html,%,$@)" "$<" -o "$@"
 
 endif