gtk-doc.make's install-data-local target checks if there are files in  
$(srcdir)/html/, proceding to install them in the system if some are  
found.  It also creates the target directory where they will be installed  
(obviously).  
  
The problem is that the directory is created before any check is done, so  
an empty directory will be created in the system even if nothing will be  
installed into it.  
  
This does not cause more problems that beeing "ugly", specially when 
packaging software that uses this piece of make code.  The package (in 
pkgsrc) will have to handle the empty directory in a special way, for no 
real reason. 
  
The fix I attached moves the mkinstalldirs call inside the conditional.