Better fix for wxX11 linking problems than r74499.

Fix wxX11 without breaking wxGTK/Windows makefiles generation.

Closes #15327.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74510 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775