Handle windows paths nicely for cmake

Make sure paths handed off to cmake (in this case `catkin/cmake/em/order_packages.cmake.em`) are given proper cmake paths (always forward slashes), otherwise cmake falls over trying to parse invalid escape sequences and you get the error of the kind below.

```
-- Using CATKIN_TEST_RESULTS_DIR: C:/work/ws/build/test_results
-- catkin 0.5.64
-- BUILD_SHARED_LIBS is on
CMake Error at C:/work/ws/build/catkin_generated/order_packages.cmake:58 (set):
  Syntax error in cmake code at

    C:/work/ws/build/catkin_generated/order_packages.cmake:58

  when parsing string

    C:\Python27\lib\site-packages\catkin_pkg\templates\groovy\metapackage.cmake.in

  Invalid escape sequence \P
Call Stack (most recent call first):
  catkin/cmake/catkin_workspace.cmake:35 (include)
  CMakeLists.txt:51 (catkin_workspace)


-- Configuring incomplete, errors occurred!
```