[BuildTools] --enable-doscompile, msvc, mingw

[Lou Hafer]

Folks,

	With a bit of prompting from Philip Walton, I've started to get serious
about building dylp with msvc. At Philip's suggestion, I started out by trying 
to build by running bash from the command line window provided at msvc 
installation.

	There are some interesting issues, and one of them is getting configure
to abandon its GCC preferences. There are already a number of hooks in coin.m4, 
but they're not all coordinated. I'd like to propose the following: Instead of 
just --enable-doscompile, allow it to take an argument from the set msvc, mingw, 
cygwin. We can choose any of the three to be the default when no argument is 
given. Specifically:

  --enable-doscompile=msvc	Will select cl/link
  --enable-doscompile=mingw	Will select gcc/g++/ld, plus -mno-cygwin
  --enable-doscompile=cygwin	Will select gcc/g++/ld and use cygwin

Other options (compile & link flags, other stuff I haven't thought of yet) can 
be set appropriately. Users would still be able to override.

	Thoughts?
	
							Lou