Adding documentation on how to build and run on top of EESSI without EB

[hvelab]

To follow up the short-term solution in issue https://gitlab.com/eessi/support/-/issues/48

[ocaisa]

@hvelab This is a good start. I think it is probably worth explaining the weird addition of the GCCcore path to LD_LIBRARY_PATH

[ocaisa]

I think it's probably worth adding a big fat warning at the beginning that this is really currently only for those who have a good understanding of what LD_LIBRARY_PATH is, what a runtime linker is and how library dependencies are resolved on Linux:

Suggested change

	!!! warning
	We are working on a module file that should make building on top of EESSI (without using EasyBuild)
	more straightforward, particularly when using `Autotools` or `CMake`. Right now, it is a little convoluted
	and requires you to have a decent grasp of
	* What a runtime linker (`ld-linux*.so`) is and does
	* How to influence the behaviour of the runtime linker with `LD_LIBRARY_PATH`
	* The difference between `LIBRARY_PATH` and `LD_LIBRARY_PATH`
	As such, this documentation is intended for "experts" in the runtime linker and it's behaviour,
	and most cases are untested. Any feedback on this topic is highly appreciated.

[ocaisa]

@hvelab I learned about LD_RUN_PATH today, that is used at link time and removes the need for LD_LIBRARY_PATH at runtime. It means you can set

LD_RUN_PATH=$LIBRARY_PATH:$EBROOTGCCcore/lib64

at compile time only, and the application should run just fine afterwards. You can probably even have export a shell function that does that for you (bash only)

[hvelab]

@ocaisa Which way do you think it's the best to add this piece of information? Should we replace the runtime part line where

export LD_LIBRARY_PATH=$LIBRARY_PATH:$EBROOTGCCcore/lib64

is mentioned or should we add it in a separate section as an untested feature?

[ocaisa]

It's already tested in the sense that we've tried it and it works. I imagine there are probably corner cases where it will not work. I would be inclined to replace the use of LD_LIBRARY_PATH as this is a much more straightforward solution.

[hvelab]

just pushed a commit with these changes: ed0c5dd