How do I *conditionally* add to INSTALL_DIRS ?

[atoz]

I have added some install options to configure, for example ...

--with-man-pages
--with-readme

and some others

I then tried to get automake to add the relevant instructions via  
automake.am INSTALL_DIRS, like so ...

# man pages
if test -n "${with_man_pages}"; then \
        INSTALL_DIRS += $(foomandir) \
fi

# documentation
if test -n "${with_readme}" -o -n "${with_license}" -o -n "$ 
{with_credits}" -o -n "${with_sgml_docs}"; then \
        INSTALL_DIRS += $(foodocdir) \
fi

but all this did was copy the above VERBATIM into Makfile.in and of  
course that doesn't install the files then.

any ideas/hints what I need to do to get automake to create the  
correct instructions in the makefile, but only if the respective  
options have been set?

thanks a lot in advance