HPCC-15306 only set "-stdlib=libc++" on OS X

Add "-Wno-unknown-warning-option" since "-Wno-inconsistent-missing-override"
is not defined in legacy clang.

cmake_modules/commonSetup.cmake

@@ -397,11 +397,14 @@ IF ("${COMMONSETUP_DONE}" STREQUAL "")
       endif ()
     endif ()
     if (CMAKE_COMPILER_IS_CLANGXX)
-      SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++ -Werror=logical-op-parentheses -Werror=bool-conversions -Werror=return-type -Werror=comment")
+      SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=logical-op-parentheses -Werror=bool-conversions -Werror=return-type -Werror=comment")
+      if (APPLE)
+        SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
+      endif ()
       SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}  -Werror=bitwise-op-parentheses -Werror=tautological-compare")
       SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}  -Wno-switch-enum -Wno-format-zero-length -Wno-switch")
       SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}  -Qunused-arguments")  # Silence messages about pthread not being used when linking...
-      SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}  -Wno-inconsistent-missing-override")   # Until we fix them all, whcih would be a huge task...      
+      SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}  -Wno-inconsistent-missing-override -Wno-unknown-warning-option")  # Until we fix them all, whcih would be a huge task...
       if (CLANG_VERSION VERSION_GREATER 3.6 OR CLANG_VERSION VERSION_EQUAL 3.6 OR APPLE_CLANG_VERSION VERSION_GREATER 6.0)
         SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-pointer-bool-conversion")
       endif()