When
1. Spack installs libtool,
2. system libtool is installed too, and
3. system automake is used
Spack passes system automake's `-I <prefix>` flag to itself, even though
it's a default search path. This takes precedence over spack's libtool
prefix dir. This causes the wrong `libtool.m4` file to be used (since
system libtool is in the same prefix as system automake).
And that leads to error messages about incompatible libtool, something
something LT_INIT.
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