atk/Makefile.am: Stop reading atk.def

Since 41442d82 we no longer generate atk.def, but we were still reading
it when linking libatk on MinGW.  This was causing a build failure:

      CC       atkwindow.lo
      CC       atk-enum-types.lo
    /usr/bin/i686-w64-mingw32-windres atk.rc atk-win32-res.o
      CCLD     libatk-1.0.la
    libtool: link: symbol file `atk.def' does not exist
    make[3]: *** [libatk-1.0.la] Error 1

https://bugzilla.gnome.org/show_bug.cgi?id=730859
MSVC 2010+ Projects: Update "Installation" Process

Currently, due to the way that Visual Studio 2010+ projects are handled,
the "install" project does not re-build upon changes to the sources, as it
does not believe that its dependencies have changed, although the changed
sources are automatically recompiled.  This means that if a part or more
of the solution does not build, or if the sources need some other fixes
or enhancements, the up-to-date build is not copied automatically, which
can be misleading.

Improve on the situation by forcing the "install" project to trigger its
rebuild, so that the updated binaries can be copied.  This does trigger an
MSBuild warning, but having that warning is way better than not having an
up-to-date build, especially during testing and development.
Merge branch 'master' of git://git.gnome.org/atk