reftest: Build a shared lib on win32

reftest: Build a shared lib on win32

Windows needs a shared library to link the modules against, otherwise
the undefined symbols make it not work.

So build a shared library on Windows.

We don't want a library elsewhere, as that just complicates things, so
we only make the library shared on Windows and keep it as a noinst
library otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=736338