Plug-ins on Windows

[Kyle Girard]

> The attached patch still has some rough edges, like missing main gcc
> ChangeLog, missing documentation, no dso building implemented (should  
> eventually work both in-tree to be installed in the plugin dir and
> out-of-tree with a previous built compiler).
> 

Well, I definitely owe you some beer.  The gcc I am using isn't trunk
but the majority of the changes applied cleanly.  I had to back port
some changes from trunk to my version and do a little extra hack or two
to get it to work on Windows but it works like a charm!

On Windows, when you configure gcc it doesn't set ENABLE_PLUGIN because
dynamically linked plugins are (rightfully so) unsupported and
LINK_PLUGIN_NAMES doesn't get set by configure.  Therefore I had to add
LINK_PLUGIN_NAMES define to the compile options for opts.o in the
gcc/Makefile.in.  The exact same options your patch added for plugin.o,
after that and fixing a missing return type and semi colon in plugin.c
everything works like a charm.  You definitely saved me quite a bit of
time thanks again!

I think if this patch was polished up it could be at least a short term
solution for plugin support on Windows.  Plus it's a cool feature on
other platforms as well.

Cheers,

Kyle