Remove the PREFIX from generated binaries

[SylvainCorlay]

We pass the install prefix as a compile time definition for the tag files, making relocation less easy (via the XCPP_TAGFILES_DIR compile time definition).

We should rather use xtl::prefix_path at runtime to prevent the need for binary relocation.

This issue also applies to xeus-cling.

[vgvassilev]

I was thinking if we could rely on the environment here somehow. That way we can have one less dependency.

[JohanMabille]

This function has been moved to xeus, so we should be able to use it without reintroducing the xtl dependency.
When this is done, the config file should be restored to its previous version instead of being configured by CMake.