Add Autotools build system (automake, autconf)

This adds the familiar autotools build system and associated documentation
for Linux, FreeBSD, Mac, MinGW, and Cygwin.  The old Makefiles have been
kept, and where appropriate have been renamed Makefile-manual.

Thanks to Peter Stuge, Ludovic Rousseau, Xiaofan Chen, Alex Dupre, and
Segher Boessenkool for providing testing, review, and suggestions, and to
Ludovic Rousseau for providing patches which contributed to this commit.

bootstrap

@@ -0,0 +1,2 @@
+#!/bin/sh -x
+autoreconf --install --verbose --force