The check for `wxWidgets_LIBRARIES` variable, introduced by e8b77084, fails
with the `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
thing for `wxWidgets_INCLUDE_DIRS`.