Teach tablegen() macro to check needed variables

This macro depends on several variables to be set in the calling
context.  Check them and report an error if they are not set.
Without this, custom commands may be silently specified that
will fail at build time.

Patch by Brad King.
Windows/Path.inc: Move <shlobj.h> after "Windows.h" for some API available.

  I found that swapping the order of some header files helped fix a
  build issue that we're seeing on mingw32. Without the swap, windows.h
  was being included before _WIN32_WINNT was being defined and the
  CreateHardLinkW function was #ifdef'd out.

  It looks like the header is mainly used to get the SHGetFolderPathW
  function, so I don't think that there'll be much fallout from the
  switch.

Suggested by Alex Crichton. Thanks!