Revert hack that broke things badly on Windows

Revert hack that broke things badly on Windows

This should fix bug #628952.

Don't include glib/gdatasetprivate.h directly. Especially don't define
GLIB_COMPILATION when doing that, as that causes breakage on Windows
because of the variable dllimport/dllexport stuff in gtypes.h that
checks GLIB_COMPILATION. That macro really should be defined only when
compiling code that goes into the libglib DLL. Otherwise the compiler
thinks that variables that should be imported from libglib are
actually defined in the code being compiled.

Just call g_atomic_pointer_get() as such, don't bother with
G_DATALIST_GET_FLAGS.

Signed-off-by: Tor Lillqvist <tml@iki.fi>