gtkmm: minor cleanup
gtkmm: minor cleanup

* it is enough to inherit gnomebase and distro_features_check
* GNOME_COMPRESS_TYPE = "xz" is default for very long time

Signed-off-by: Andreas Müller <d72a15aaaca3a7afa855175c72dc8d2e05931e32@gmail.com>
Signed-off-by: Armin Kuster <525edcd226fb27e7f47fbb319a38fe989c677548@gmail.com>

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