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/branches/WX_3_0_BRANCH@75712 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775