By default, libfuse install helper programs like `fusermount3`, which
are mostly useless if not installed with setuid (that is, `+useroot`).
However, their presence makes it complicated to use globally installed
versions, which can be combined with a Spack-installed FUSE library.
In particular, on systems that have a setuid fusermount3 binary, but no
libfuse-dev installed, it is nice to be able to build libfuse with Spack, and
have it call the system setuid executable.
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