`DYLD_LIBRARY_PATH` can frequently break builtin macOS software when
pointed at Spack libraries. This is because it takes *higher* precedence
than the default library search paths, which are used by system software.
`DYLD_FALLBACK_LIBRARY_PATH`, on the other hand, takes lower precedence.
At first glance, this might seem bad, because the software installed by
Spack in an environment needs to find *its* libraries, and it should not
use the defaults. However, Spack's isntallations are always `RPATH`'d,
so they do not have this problem.
`DYLD_FALLBACK_LIBRARY_PATH` is thus useful for things built in an
environment that need to use Spack's libraries, that don't set *their*
RPATHs correctly for whatever reason. We now prefer it to
`DYLD_LIBRARY_PATH` in modules and in environments because it helps a
little bit, and it is much less intrusive.
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