Adding command line parameters to configure

[Huntsman, CD Clayton @ IS]

I am trying to extend the pathing conventions so that configure can be
called with an additional path variable. I can pass the built in paths
on the command line, e.g. configure --sysconfdir=/etc but I cannot find
a way to pass in variables which are extensions of the predefined ones.
 
Since I am doing cross-system development, I would like to be able to
specify the init script directory from the command line, since the
product is build on the target machine from a script which has its
variables assigned on a command line generated on another machine. So,
what I wanted to do was have a sysinitdir variable so that I could write
configure --sysinitdir=/etc/init.d. 
 
The only way I have found to extend configure's command line is through
the AC_ARG_ENABLE. But, this macro requires a syntax of the form
enable-variable or disable-variable, which is not really what I want.
 
The question is, is there some sort of hook so that I can expand the
option set to configure, that has command line checking, etc like the
built-in path variables do?