How to retrieve an absolute path in this case?

[ant1scene]

Hello. 

I need lookup config's path in application. 
Tried as is: 

--configure.in (fragment)
AC_DEFINE_UNQUOTED(PREFIX, $prefix, [Install prefix])
AC_DEFINE_UNQUOTED(SERVER_CONFIG_FILE, "${sysconfdir}/foo.conf",
        [Location of the config file])
--
but, after running 'configure' I'm getting such config.h:
#define SERVER_CONFIG_FILE "${prefix}/etc/foo.conf"
#define PREFIX /usr

what to do to retrieve ${prefix} in SERVER_CONFIG_FILE?

P.S. Sorry for my bad English :\