[PD-dev] minor header change to make externals compile on all versions of Pd

[Hans-Christoph Steiner]

If the pd headers did this, then externals could compile on both <=  
0.36 and 0.37 without changes:

/* Copyright (c) 1997-1999 Miller Puckette.
* For information on usage and redistribution, and for a DISCLAIMER OF  
ALL
* WARRANTIES, see the file, "LICENSE.txt," in this distribution.  */

#ifndef __m_pd_h_

...

#define __m_pd_h_
#endif /* __m_pd_h_ */

This is most needed on m_pd.h, but I think it couldn't hurt in the  
other headers that are commonly used by externals:
m_imp.h, s_stuff.h, g_canvas.h.  Thomas Grill has had this change in  
the CVS devel_0_3? version for a while now, so I think its well tested.

.hc


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