Fix `wx-config --libs` in monolithic build.

Don't include non-existent libraries (which are built as part of the main
monolithic one) in this case.

Closes #15567.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75713 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Don't keep dangling pointers in other wxRibbonButtonBar members neither.

This should have been part of r75710: after deleting the button, also reset
any stored pointers to it.

Closes #15909.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75714 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775