Subject: glibmm-2.64 propagates two different versions of libsigc++

From: Leo Famulari <[email-masked]>
Date: Wed, 15 Dec 2021 13:40:09 -0500

glibmm-2.64 has this:

------
     (propagated-inputs
      (modify-inputs (package-propagated-inputs glibmm)
        (prepend libsigc++-2)))))
------

And, the glibmm package also propagates libsigc++.

So:

------
$ guix show [email-masked] | grep libsigc
dependencies: [email-masked] [email-masked] [email-masked] [email-masked]
+ [email-masked] [email-masked] [email-masked] [email-masked] [email-masked]
------

It propagates both versions of libsigc++, and thus cannot be installed
into a profile. 1785 packages depend on glibmm-2.64.

This breaks synfig, which propagates glibmm, as requested in its 'synfig.pc'
pkg-config file.