add optional files to install

[samoht]

Sometime, depending on the optional packages already installed, some binaries or libraries are not produced.

It would be good to be able to mark them as optional (or to not fail if some files are not there).

[tuong]

We can split P.install to several files :

• P.install which contains all the files that must be installed
• P_opt.install which contains the files for optional package P_opt.

[samoht]

This could work, but we need to handle native-code/bytecode files as well. I guess it's more a flag we should add to files in .install instead of creating an other file for this (let's try to keep it simple).