We only define NEED_SOLARIS_BOOLEAN in an environment where the solaris
compat boolean stuff will be defined by this point (eg, when we're
building zfs.c in libsa), but not in other environemnts (like when we're
building mkimage and variousl stand-alone boot loaders that don't use
libsa for space reasons. efi/boot1 also avoids libsa, but any space
reasons for that are long since gone (space no longer matters there),
but rewriting it isn't a priority. This allows zfsimpl.h to now build in
both environments.
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