libmerged works on Windows

[Matúš Kukan]

Hi there,

I forgot to write about --enable-mergelibs build working on Windows now too.
Since 4c251ddd72af765dd9e665150176c2d931322b9c - patch only in master,
simple to backport if it makes sense.

I think it would be good to release with that option - should make (at
least) first start faster.
If anyone know how and can produce some numbers on Windows, that would be great.
Bjoern had some numbers for linux in the past IIRC.

Also, it would be great if some tinderboxes could build with mergelibs
(switching the option requires make clean :-( [or you need to know
what to clean exactly])
So far, I know only about Linux-x86_64_32-callgrind using that.

Hoping it will be useful,

Matus