huh? you mean theres a m_pd.h in src/ or build/OS? sounds like a bad idea..
exactly 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?
best greetings, Thomas