Add macro to add compile options to ROOT

The intended use of this macro is to add compile options that are not
requirements of ROOT, like options to ignore certain warnings. However,
since the option names depend on the compiler, check each flag before
adding it to the list, so that only supported flags are added.

cmake/modules/RootNewMacros.cmake

@@ -1419,6 +1419,20 @@ function(ROOT_ADD_C_FLAG var flag)
   endif()
 endfunction()
 
+#----------------------------------------------------------------------------
+# ROOT_ADD_COMPILE_OPTIONS(flags)
+#----------------------------------------------------------------------------
+macro(ROOT_ADD_COMPILE_OPTIONS flags)
+  foreach(__flag ${flags})
+    check_cxx_compiler_flag("-Werror ${__flag}" __result)
+    if(__result)
+      add_compile_options(${__flag})
+    endif()
+  endforeach()
+  unset(__flag)
+  unset(__result)
+endmacro()
+
 #----------------------------------------------------------------------------
 # find_python_module(module [REQUIRED] [QUIET])
 #----------------------------------------------------------------------------