chicken-install checks for installed files even when -n is passed

[Vasilij Schneidermann]

I've discovered chicken-install -n for fetching dependencies and building programs without actually installing them. Unfortunately chicken-install refuses building programs if such a program has been installed before, despite passing an option to not install anything.

I've attached a simple fix that moves the check right before actually installing the files.

[Vasilij Schneidermann]

Clarification: The conflict situation happens if you have two different eggs that provide the same file. Suppose you install an elephantdb egg providing a createdb program and build a personal-stuff egg providing a createdb program, too. Installing the former works fine, building the latter doesn't. The change I propose here would allow building the latter, but still prevent you from installing the latter.

[sjamaan]

Note that the egg's name must be different from the egg that originally installed the program or library that you're building in the new egg.

[Vasilij Schneidermann]

Fixed on master