troubles with conditional install using automake

[Guillaume Rousse]

I'm trying to implement conditional installation of stuff with 
non-standard location, such as init-script. The basic idea is not to 
install them unless the user specify a location when running ./configure

However, it doesn't work with automake conditional
in configure.ac:
AC_ARG_WITH(
        initrddir,
        [  --with-initrddir=DIR    path to install init script],
        initrddir=$withval,
        initrddir=""
)
AC_SUBST(initrddir)
AM_CONDITIONAL(WANT_INITRD, test "$initrddir" != "")

in makefile.am, either:
if WANT_INITRD
        initrd_SCRIPTS = parserd
endif

In this situation, I don't have any installation target for initrd 
scripts in generated Makefile

or:
if WANT_INITRD
        INITRD = parserd
else
        INITRD =
endif
initrd_SCRIPTS = $(INITRD)

In this situation, I have an installation target for initrd scripts in 
generated Makefile, using $(initrd_SCRIPTS) as dependency. However, 
initrd_SCRIPTS is defined to $(INITRD) at the beginning of the Makefile, 
while INITRD is defined at the end of the Makefile, with other 
conditional variables, and thus appears empty when it is evaluated.

So, is there any solution to my problem before i consider using autoconf 
substitutions instead ?

BTW, I'm using autoconf 2.53 and automake 1.8

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