cmake: add XL C/C++ support (theoretically)

Author: torbjoernk

CMakeLists.txt

@@ -35,6 +35,14 @@ elseif(${CMAKE_CXX_COMPILER_ID} MATCHES Clang)
     else()
         message(FATAL_ERROR "No C++11 support for Clang version. Please upgrade Clang to a version supporting C++11.")
     endif()
+elseif(${CMAKE_CXX_COMPILER_ID} MATCHES XL)
+    # NOTE: This branch is not tested yet (in theory it should work)
+    check_cxx_compiler_flag(-qlanglvl=extended0x HAVE_STD11)
+    if(HAVE_STD11)
+        add_to_string_list("${CMAKE_CXX_FLAGS}" CMAKE_CXX_FLAGS "-qlanglvl=extended0x -qwarn0x")
+    else()
+        message(FATAL_ERROR "No advanced standard C++ support of your IBM XL C/C++ compiler (-qlanglvl=extended0x not defined).")
+    endif()
 else()
     message(FATAL_ERROR "Don't know how to check C++11 compatibility with compiler '${CMAKE_CXX_COMPILER_ID}'")
 endif()