m_pd.h didn't really change from 0.37 to 0.38, not sure about m_imp.h / g_canvas.h, though ... butthey are private headers and shoudn't be used by externals, althought e.g. flext or cyclone _are_ using them. (so they are de facto public) ...
they (or a part of their contents) should be made public as soon as possible anyway, because how would one program PD gui objects (like Yves') without them?
not at all ...
... just kidding .... t