Cygwin: Export all symbols with ENABLE_EXPORTS

The ENABLE_EXPORTS property exports all symbols from executables on
UNIX-like platforms, typically for use by plugins.  Honor this behavior
on Cygwin.  See issue #10122.

Modules/Platform/CYGWIN-GNU.cmake

@@ -45,6 +45,7 @@ macro(__cygwin_compiler_gnu lang)
     SET(CMAKE_${type}_LINK_DYNAMIC_${lang}_FLAGS "-Wl,-Bdynamic")
   ENDFOREACH(type)
 
+  set(CMAKE_EXE_EXPORTS_${lang}_FLAG "-Wl,--export-all-symbols")
   # TODO: Is -Wl,--enable-auto-import now always default?
   set(CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS "${CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS} -Wl,--enable-auto-import")
   set(CMAKE_SHARED_MODULE_CREATE_${lang}_FLAGS "${CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS}")