Add CMake support for export decoration
Add CMake support for export decoration

Teach `lcm_wrap_types` a new `C_EXPORT` option to leverage the newly
added ability of lcmgen to do export decoration. This is intended to be
used with `generate_export_header`.

bluesquall/lcm,lcm-proj/lcm,lcm-proj/lcm,adeschamps/lcm,adeschamps/lcm,bluesquall/lcm,adeschamps/lcm,bluesquall/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,adeschamps/lcm,bluesquall/lcm,lcm-proj/lcm,adeschamps/lcm,adeschamps/lcm,lcm-proj/lcm,bluesquall/lcm,bluesquall/lcm,bluesquall/lcm,adeschamps/lcm,adeschamps/lcm