Fixed exporting of symbols.
Looks like `-export-symbols-regex` doesn't work correctly with `libtool` and C++ code, so use `visibility` attribute instead to mark symbols as exported.
Remaining are a couple of `std::` symbols I can't get rid of, and some debug draw functions used by `sherlock265`.
Needs autoconf macro to check visibility support in compiler.
Merge pull request #72 from fancycode/exports_visibility

Fixed exporting of symbols.