bug#16014: Inconsistent loading of image libraries on Windows

[Juanma Barranquero]

Running consecutive instances of emacs.exe from the very same command
line, it sometimes loads libpng, and sometimes it doesn't.

The recipe is as simple as opening a cmd shell and running "emacs -q",
exiting the Emacs instance and repeating these steps a few times. Most
of the time Emacs does not load libpng, though sometimes it does.

In the images I've done M-x list-dynamic-libraries to show the
difference, though the lack of the Emacs logo is quite enough to see
when the library has been loaded and when it hasn't.

This has started happening quite recently, though I'm not sure when.

bug-bad.png
Description: PNG image

bug-good.png
Description: PNG image