Fix for issue syoyo/tinyobjloader#217
Fix for issue syoyo/tinyobjloader#217

Set a target property WINDOWS_EXPORT_ALL_SYMBOLS to true every time shared libraries are built.

This enables the generation of symbols to be exported in Windows DLLs.

syoyo/tinyobjloader,syoyo/tinyobjloader,syoyo/tinyobjloader,syoyo/tinyobjloader