use export all symbols on cygwin

Modules/Platform/CYGWIN.cmake

@@ -1,4 +1,4 @@
-SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared")
+SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared -Wl,--export-all-symbols -Wl,--enable-auto-import")
 SET(CMAKE_DL_LIBS "-lgdi32" )
 SET(CMAKE_SHARED_LIBRARY_PREFIX "lib")
 SET(CMAKE_SHARED_LIBRARY_SUFFIX ".dll")