FindwxWidgets: Fix wxWidgets_LIBRARY_DIRS on Cygwin/MSYS
The check for
wxWidgets_LIBRARIES variable, introduced by e8b77084, fails
wx-config script generated by an MSYS build, because
wxWidgets_LIBRARY_DIRS is in POSIX style.
This commit fixes the problem with
wxWidgets_LIBRARY_DIRS by converting it to
Windows style. The actual code were copied from bf643286, which does the same