Tweaking autotools-based buildsystem for W32 compatibility

[Ethan Blanton]

LRN spake unto us the following wisdom:
> On 25.04.2014 17:17, Ethan Blanton wrote:
> >> 
> >> -libjabber_la_LDFLAGS = @PLUGIN_LDFLAGS@
> >> +libjabber_la_LDFLAGS = -no-undefined
> > 
> > This seems bogus to me.  What's the deal?
>
> PLUGIN_LDFLAGS resolves into "-no-undefined -avoid-version"
> This changes it to just "-no-undefined", because i like versioned shared
> libraries.

OK.  If we want to make this change (which I do not think we do; if we
want to version plugins we probably want to do it differently), it
needs to be by way of changing PLUGIN_LDFLAGS, not hard-coding linker
options in the various Makefile.am files.

Ethan