Exclude the XL compiler from the hidden-visibility test.

Modules/GenerateExportHeader.cmake

@@ -146,7 +146,10 @@ macro(_test_compiler_hidden_visibility)
     endif()
   endif()
 
-  if (NOT GCC_TOO_OLD AND NOT WIN32)
+  # 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)
     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)