fix build on MinGW-w64 and MSVC

[Adsun701]

This sets rsync_EXPORTS on Windows to prevent build failure. In addition, missing LIBRSYNC_EXPORT types are added to certain variables in librsync.h to prevent redefiniton errors on MSVC, and dlls are correctly installed into the bin directory.

[dbaarda]

Thanks for doing this... I don't have a system where I can test this platform.

Theres on thing in the CMakeLists.txt that I think is not required, but everything else looks fine.

Can you fix and/or confirm if its needed?

[dbaarda]

The description will also need updating.

[dbaarda]

We don't want rdiff statically linked, we want it dynamically linked against the rsync library.

If that doesn't work for some reason, there may be other problems we need to fix for MinGW and/or MSVC.

[dbaarda]

I think the changes to rdiff.c are wrong and should not be required.

If it won't compile without these changes, there must be something else going wrong somewhere.

[dbaarda]

Thanks for doing this and being patient with all the back-and-forth to get this right.