The build process in exp/lpj_compile.csh expects to find the
to-be-used LPJmL version in a subdirectory src/land_atlantes/LPJmL .
That used to be a SVN external reference, pointing the SVN-hosted
development version of LPJmL, which got checked out automagically with
the POEM source. However, current (from 2019 on) LPJmL versions are now hosted
on GIT repositories, and different users might want to use different
LPJmL development branches. Thus, you need to check out your favourite
LPJmL version by hand, either into a subdirectory named LPJmL,
or create a symbolic link nmed LPJmL, which points to your
checked-out LPJmL source code tree.

Before invoking the POEM compile script, you must invoke the configure script
inside your LPJmL source directory, so that Makefile.inc is created.