Add build of ovsext.sln using MSBuild

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>