detect the windows um include directory

[andrewrk]

Previous versions of Windows had a path like this: C:\Program Files (x86)\Windows Kits\10\Include\10.0.10150.0\ucrt

And that had all the .h files in it. However every ucrt newer than that, including the newest one, look like this: C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0, with subdirs:

• cppwinrt
• shared
• ucrt
• um
• winrt

The um and shared directories have .h files that are desired when linking libc. For example, if you try to depend on GLFW, it's going to try to #include <GL/gl.h> (found in the um dir), which itself will #include <winapifamily.h> (found in the shared dir).

This pull requests adds the um and shared directories to the list of include directories when targeting windows and linking libc.