This commit makes four changes:
1. Adds the ts_ prefix to some functions that were missed in the last
pass
2. Adds static to some variables that were confined to a single file but
missing it
3. Adds the ability to disable using "hidden" visibility by default by
setting the USE_DEFAULT_VISIBILITY variable
4. Switches the prefix-check in travis to use the flag defined in 3 so
that the checker now checks all non-static symbols we define