Adding GnuTLS to the Windows port

[Juanma Barranquero <lekktu@gmail.com>]

I've been working on compiling Emacs on Windows with GnuTLS support.

It works, but there are a few questions that should be decided before
I dig deeper into it.

Currently I can link Emacs against the static library libgnutls.a, or
use libgnutls.dll.a and have Emacs depend on libgnutls-26.dll (which
in turn brings libgcrypt-11, libpgp-error-0 and libtasn1-3).

Options:

1) Static linking
    Pros: no failure because of missing DLLs
    Contras: 5+MB, which won't be used by most Windows users; Emacs
tied to a specific version of GnuTLS.

2) Dynamic linking
    Pros: update of GnuTLS possible without recompiling
    Contras: the executable depends on the DLLs, even when dumping, so
failure to load if they are unavailable.

3) Dynamic loading (the same system used on image.c for the image libraries)
    Pros: most flexible; Emacs can run without the DLLs.
    Contras: code changes in gnutls.c to use function pointers (like image.c).

IMO, 3 is the best option, but it does not make sense to implement it
until the GnuTLS interface has stabilized, which I think it's a bit
far off yet. Opinions?

An additional question related to 3: it would make sense to rename
`image-library-alist' to `dynamic-library-alist' (with an obsolete
alias `image-library-alist'), move it to some other suitable .c file
(perhaps emacs.c?) and rewrite its doc to turn it into a more generic
facility.  It certainly would be handy to use it for (gnutls .
"libgnutls-26.dll"), and seems like a logical step.

BTW, the same question will have to be answered sooner or later for
xml.c / libxml2 and dbus.

    Juanma