Constrain CFLAGS for ZFS: don't add anything globally. Add the includes
to only the files that need them. Add -DHAS_ZSTD_ZFS to zfs.c (which
includes zfsimpl.c which includes zfssubr.c both of which need this
defined). Also add it to efi/boot1/Makefile since zfs_module.c also
includes zfsimple.c.
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