Allowing overriding of libraries in stack #88
Description
In 2021.03 we introduced the use of /opt/eessi/lib and /opt/eessi/2021.03/lib as trusted directories by the compatability layer glibc. This means that packages linked against that glibc will look in those directories by default, if they don't find their libraries elsewhere. I've tested this for the GPU drivers required for CUDA support and it works since the libraries are not to be found elsewhere in the EESSI environment.
What we discussed as well though, was being able to override EESSI libraries using this approach. Since we use rpath in EESSI, this means that we would need to inject these directories first in the the search paths of rpath to enable this. This requires some tweaking of the EB rpath wrappers to facilitate this. EB currently does not support such tweaking and we, therefore, don't have that implemented in the 2021.03 build scripts. Such a change would, for example, give sites the ability to override the EESSI MPI implementation with a system-specific ABI compatible version.