Setting a default path with autoconf

[Will Newton <will@misconception.org.uk>]

I have a package that really needs to have a configurable location of it's 
configuration files. This would be possible by putting them in $sysconfdir, 
but for some reason I am yet to come close to understanding, $sysconfdir is 
set to $prefix/etc which is not a good place to put config files in the 
general case, so in adding configurability, I break the build for the average 
user who justs wants to type ./configure without arguments.

Is there a way of setting the default value of an argument to configure? It 
seems that argument processing is forced before it is possible to set the 
value of a variable.

Or if someone can explain to me another way of easily configuring a config 
file path from configure.

Thanks,