Fix the fix, make pi a normal lisp var (Closes bug #176)

Fix the fix, make pi a normal lisp var (Closes bug #176)

This change reverts the previous one, and then goes about closing bug 176
in a different way.  Namely, by changing Vpi from being a
DEFVAR_CONST_LISP to a DEFVAR_LISP. IOW, a plain ordinary variable.

	* src/ent/ent-mpfr.c (vars_of_ent_mpfr): Set Vpi to be a normal
	lisp variable instead of a constant so packages like Calc can
	let-bind them for their own evil and misguided purposes. :-)
	Vbigfr_pi -> Vpi

	* src/ent/ent-mpfr.h: Vbigfr_pi -> Vpi

	* lisp/float-sup.el: Revert previous change.

Signed-off-by: Steve Youngs <steve@sxemacs.org>