zcat is broken when any seamless compression option but gz is set

[Thomas De Schampheleire]

Hello,

When setting no seamless compression option
(ENABLE_FEATURE_SEAMLESS_xxxx) at all, then zcat works fine.

When enabling at least one such option, e.g.
ENABLE_FEATURE_SEAMLESS_XZ, but not ENABLE_FEATURE_SEAMLESS_GZ, then
zcat is suddenly broken with following output:

    $ ./busybox zcat /proc/config.gz
    zcat: no gzip/xz magic

When additionally enabling ENABLE_FEATURE_SEAMLESS_GZ, zcat is working again.

This behavior is odd from the user perspective. One does not expect
that enabling seamless xz compression will break something else like
zcat.

Would it be possible to either fix this behavior, or prevent such
'invalid' configuration?

Thanks,
Thomas