Elevate "Fill" from Magic Tool to real Tool

Replace the somewhat-recently-added "nothing" item in the tool bar
with the "Fill" tool, which has been converted back from a Magic tool
to a regular tool.

Also, change bash "==" to sh "=" in "if [ ... ]" tests in Makefile.

Also, link to math library (via "-lm") to make sure "max()" is available to
magic tools.

Also, add missing mention of "--newcolorslast" and "--newcolorsfirst" to manpage.

src/fill.h

@@ -0,0 +1,37 @@
+/*
+  fill.h
+
+  Fill tool
+  Tux Paint - A simple drawing program for children.
+
+  Copyright (c) 2002-2019 by Bill Kendrick and others; see AUTHORS.txt
+  bill@newbreedsoftware.com
+  http://www.tuxpaint.org/
+
+  Flood fill code based on Wikipedia example:
+  http://www.wikipedia.org/wiki/Flood_fill/C_example
+  by Damian Yerrick - http://www.wikipedia.org/wiki/Damian_Yerrick
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program; if not, write to the Free Software
+  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+  (See COPYING.txt)
+
+  Last updated: September 12, 2019
+  $Id$
+*/
+
+#include "SDL.h"
+
+void do_flood_fill(SDL_Surface * canvas, int x, int y, Uint32 cur_colr, Uint32 old_colr, int * x1, int * y1, int * x2, int * y2);
+