wxPLViewer: definitive fix for the rt linking issues.
wxPLViewer: definitive fix for the rt linking issues.

Tested by Alan W. Irwin <airwin@users.sourceforge.net> on Linux using
the test_c_xwin, and test_c_wxwidgets targets in the build tree, and
by explicit hand-testing of example 2 page 2 with -dev xwin.

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