Fix build under OS X with --disable-xrc.

Don't try to change the install names in non-existent (in this case) wxrc.

Closes #15946.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75844 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775