[master] don't use ?= for variable assignment

Solaris make says:

make: Fatal error in reader: Makefile, line 397: Badly formed macro assignment

It is fine with NetBSD's bmake and GNU gmake.

Anyways, it is not used by any target.

This was to choose the HTML-renderer.

doc/guide/Makefile.am

@@ -2,7 +2,7 @@ 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
+HTML2TXT = elinks -dump -no-numbering -no-references
 
 bind10-guide.txt: bind10-guide.html
 	$(HTML2TXT) $(srcdir)/bind10-guide.html > $@