Mkmf question - conditional installs

I have two extensions I'm working on. Both .c files (we'll call them x.c
and y.c) are in the same directory. One of these files I want to install
*only* if it isn't already installed. So, I wrote my extconf.rb file like

The best, probably, is to have an extconf.rb for each of your extension.

You have an example in bdb

Guy Decoux