The actual configure script in libxslt recognizes the --with-html-dir  
argument which is used to define the value of HTML_DIR.  Even though,  
this variable is later overriden by hardcoded values in all Makefile.am  
files using it, making the option useless.  
  
While fixing this, I've also done some improvements that will make  
packaging easier in pkgsrc, for example. Many other gnome packages  
(specially those using gtk-doc) install documentation under  
${HTML_DIR}/${PACKAGE}, beeing the first variable defined by the  
configure argument.  Even though, libxslt behaves differently, because it  
adds a trailing 'html/' to this path, resulting in undesired locations in  
our filesystem hierarchy (and probably in others).  
  
To allow all installation flavours, I've simply added another option,  
named --with-html-subdir, whose value is $(PACKAGE)-$(VERSION)/html by  
default.  
  
Note that this makes the package more consistent itself, because the html  
installation directory is defined in just one place, the configure  
script.  
  
The point is to not break the actual default behavior; that is, after  
applying the patch, libxslt will behave the same as before if no options  
are given.  But it will allow an easy customization of paths.  
  
The attached patch does this.  Note that the big chunks in the .am files  
are due to TARGET_DIR -> HTML_DIR renames, since the former becomes  
useless.