Exclude cygwin from the hidden visibility feature.

Modules/GenerateExportHeader.cmake

@@ -149,7 +149,7 @@ macro(_test_compiler_hidden_visibility)
   # Exclude XL here because it misinterprets -fvisibility=hidden even though
   # the check_cxx_compiler_flag passes
   # http://www.cdash.org/CDash/testDetails.php?test=109109951&build=1419259
-  if (NOT GCC_TOO_OLD AND NOT WIN32 AND NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES XL)
+  if (NOT GCC_TOO_OLD AND NOT WIN32 AND NOT CYGWIN AND NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES XL)
     check_cxx_compiler_flag(-fvisibility=hidden COMPILER_HAS_HIDDEN_VISIBILITY)
     check_cxx_compiler_flag(-fvisibility-inlines-hidden COMPILER_HAS_HIDDEN_INLINE_VISIBILITY)
     option(USE_COMPILER_HIDDEN_VISIBILITY "Use HIDDEN visibility support if available." ON)