Suppress -0 in the C field of lines computed by line_construct_pts().

It's not entirely clear why some PPC machines are generating -0 here, since
the underlying computation should be exactly 0 - 0.  Perhaps there's some
wider-than-nominal-precision calculations happening?  Anyway, the best way
to avoid platform-dependent results seems to be to explicitly reset -0 to
regular zero.

src/backend/utils/adt/geo_ops.c

@@ -1116,6 +1116,9 @@ line_construct_pts(LINE *line, Point *pt1, Point *pt2)
 		line->A = (pt2->y - pt1->y) / (pt2->x - pt1->x);
 		line->B = -1.0;
 		line->C = pt1->y - line->A * pt1->x;
+		/* on some platforms, the preceding expression tends to produce -0 */
+		if (line->C == 0.0)
+			line->C = 0.0;
 #ifdef GEODEBUG
 		printf("line_construct_pts- line is neither vertical nor horizontal (diffs x=%.*g, y=%.*g\n",
 			   DBL_DIG, (pt2->x - pt1->x), DBL_DIG, (pt2->y - pt1->y));