Fixed cf_compile_libstruct custom command.

Instead of specifying the absolute path of the actual binary to be
executed, the "new" ADD_CUSTOM_COMMAND supports using a target's name
directly. This fixes the cf_compile_libstruct custom command execution
on Visual Studio.
Fixed make_readme_md execution on Windows.

Windows' cmd doesn't support shebang, so we cannot call a script
directly letting the shell identifying the right interpreter. Added
explicit interpreter call.
Added VS project groups for md targets.

Added VS project groups for md targets. This feature is currently
non-working on VS12 Express, to be tested on Professional and older
editions as well.