Improved #include path handling:

* Don't use '-I-', it breaks a lot of system headers, e.g.

     #include <GL/glut.h>

  fails (when using freeglut), because /usr/include/GL/glut.h contains

     #include "freeglut_std.h"

  but /usr/include/GL/freeglut_std.h will not be found. It is a bit
  debatable if the header is broken and should use

     #include "GL/freeglut_std.h"

  instead. Anyway, a grep through the SuSE 9.1 system headers shows that
  there seems to be no real common practice, so let's play safe and don't
  use '-I-'.

* Don't use '-I .', #include stub headers "locally" instead, e.g. use

     #include "Concurrent_stub.h"

  instead of

     #include "Control/Concurrent_stub.h"

  Note that "Control" is still in the #include path, because the *.hc file
  is normally in /tmp and the stub header is in the directory where *.hs
  is. We could remove this path element, too, if the stub header would be
  copied to the directory of the *.hc file during compilation. SimonM?