[master] put the text file generation within ENABLE_MAN conditional

The FreeBSD distcheck failed during uninstall time as it attempted to
regenerate the file. Its dependency was from the same second.

Running make with debugging showed:

Examining bind10-guide.txt...non-existent...modified before source (../../../doc
/guide/bind10-guide.html)...out-of-date.

doc/guide/Makefile.am

@@ -2,11 +2,6 @@ EXTRA_DIST = bind10-guide.css
 EXTRA_DIST += bind10-guide.xml bind10-guide.html bind10-guide.txt
 EXTRA_DIST += bind10-messages.xml bind10-messages.html
 
-HTML2TXT = elinks -dump -no-numbering -no-references
-
-bind10-guide.txt: bind10-guide.html
-	$(HTML2TXT) $(srcdir)/bind10-guide.html > $@
-
 # This is not a "man" manual, but reuse this for now for docbook.
 if ENABLE_MAN
 
@@ -20,6 +15,11 @@ bind10-guide.html: bind10-guide.xml
 		http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl \
 		$(srcdir)/bind10-guide.xml
 
+HTML2TXT = elinks -dump -no-numbering -no-references
+
+bind10-guide.txt: bind10-guide.html
+	$(HTML2TXT) $(srcdir)/bind10-guide.html > $@
+
 bind10-messages.html: bind10-messages.xml
 	xsltproc --novalid --xinclude --nonet \
 		--path $(top_builddir)/doc \