Add build of ovsext.sln using MSBuild

This commit adds to the automake build system the full build required
by the forwarding extension solution.

It will help a lot in the future CI to check the full build of the project.

To configure the forwarding extension to be built one could use the following:
./configure CC=./build-aux/cccl LD="`which link`" LIBS="-lws2_32" \
    --prefix="C:/openvswitch/usr" --localstatedir="C:/openvswitch/var" \
    --sysconfdir="C:/openvswitch/etc" --with-pthread="C:/pthread" \
    --with-vstudioddk="Win8.1 Release"

Documentation will be updated in another patch.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>