[rtmpdump] Modifications for VC builds

In the process of hunterizing (https://github.com/ruslo/hunter) librtmp (and by proxy rtmpdump, etc...) I had to make some changes to get librtmp compiling for Windows under Visual Studio. Is this something that the group would like to include in the source if so what is the process for setting up a pull request. Current changes can be seen at ( https://github.com/caseymcc/rtmpdump/commit/98d920b551deebce2c06d2009b1684b2...), it hasn't been posted to Hunter yet as I need to verify the remaining builds linux, mac, android, etc... as it is running off of cmake now. Thanks, Casey