FindwxWidgets: Fix wxWidgets_LIBRARY_DIRS on Cygwin/MSYS
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
.