okay millert@

to #ifndef __cplusplus.  It turns out there is code outside of libm
that wants to use this.  I suppose that's why it is in math.h instead
of math_private.h ;-)

`math.h'.  The one in libm is more up to date but the one in the
include dir is what gets installed.

entire trees for testing anyway, I might as well do this intrusive touching
of include files now.  Added openBSD tags.