Fix some issues reported by lintian

Fix some issues reported by lintian

• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)

• Remove unused license definitions for GPL-2+. (unused-license-paragraph-in-dep5-copyright)

• Update standards version to 4.6.0, no changes needed. (out-of-date-standards-version)

• Avoid explicitly specifying -Wl,--as-needed linker flag. (debian-rules-uses-as-needed-linker-flag)

• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)

• Avoid explicitly specifying -Wl,--as-needed linker flag. (debian-rules-uses-as-needed-linker-flag)

• Set field Upstream-Contact in debian/copyright.

• Remove obsolete field Contact from debian/upstream/metadata (already present in machine-readable debian/copyright).

This merge proposal was created automatically by the Janitor bot. For more information, including instructions on how to disable these merge proposals, see https://janitor.debian.net/lintian-fixes.

You can follow up to this merge proposal as you normally would.

The bot will automatically update the merge proposal to resolve merge conflicts or close the merge proposal when all changes are applied through other means (e.g. cherry-picks). Updates may take several hours to propagate.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/libsigc++-2.0/a54b0854-3b0c-443e-bf50-afccb1ba43ac.

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/lib/debug/.build-id/fe/3b4dc1ce98d9ae99cf630089604951c7a693ef.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/8c/2ccfa8fe928c59908b8660a86ad849e7ac1a21.debug

No differences were encountered between the control files of package **libsigc++-2.0-0v5**

Control files of package libsigc++-2.0-0v5-dbgsym: lines which differ (wdiff format)

• Build-Ids: 8c2ccfa8fe928c59908b8660a86ad849e7ac1a21 fe3b4dc1ce98d9ae99cf630089604951c7a693ef

No differences were encountered between the control files of package **libsigc++-2.0-dev**

No differences were encountered between the control files of package **libsigc++-2.0-doc**

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