Make GN use_custom_libcxx work properly with sanitizers.

tools/gn/functions.cc

Index: tools/gn/functions.cc
diff --git a/tools/gn/functions.cc b/tools/gn/functions.cc
index 86043f0a73ab930c5c00fd28832e434b7a208331..7d9e669a3be9be3e608f372a1477466c9d00e7ee 100644
--- a/tools/gn/functions.cc
+++ b/tools/gn/functions.cc
@@ -344,7 +344,46 @@ const char kDeclareArgs_Help[] =
     "\n"
     "  See also \"gn help buildargs\" for an overview.\n"
     "\n"
-    "Example:\n"
+    "  The precise behavior of declare args is:\n"
+    "\n"
+    "   1. The declare_arg block executes. Any variables in the enclosing\n"
+    "      scope are available for reading.\n"
+    "\n"
+    "   2. At the end of the executing the block, any variables set within\n"

[Bons, 2015/10/02 18:55:22]

"at the end of the executing the block"...?

+    "      that scope are saved globally as build arguments, with their\n"
+    "      current values being saved as the \"default value\" for that arg.\n"
+    "\n"
+    "   3. User-defined overrides are applied. Anything set in \"gn args\"\n"
+    "      now overrides any default values. The resulting set of variables\n"
+    "      is promoted to be readable from the following code in the file.\n"
+    "\n"
+    "  This has some ramifications that may not be obvious:\n"
+    "\n"
+    "    - You should not perform difficult work inside a declare_args block\n"
+    "      since this only sets a default value that may be discarded. In\n"
+    "      particular, don't use the result of exec_script() to set the\n"
+    "      default value. If you want to have a script-defined default, set\n"
+    "      some default \"undefined\" value like [], \"\", or -1, and after\n"
+    "      the declare_args block, call exec_script if the value is unset by\n"
+    "      the user.\n"
+    "\n"
+    "    - Any code inside of the declare_args block will see the default\n"
+    "      values of previous variables defined in the block rather than\n"
+    "      the user-overridden value. This can be surprising because you will\n"
+    "      be used to seeing the overridden value. If you need to make the\n"
+    "      default value of one arg dependent on the possibly-overridden\n"
+    "      value of another, write two separate declare_args blocks:\n"
+    "\n"
+    "        declare_args() {\n"
+    "          enable_foo = true\n"
+    "        }\n"
+    "        declare_args() {\n"
+    "          # Bar defaults to same user-overridden state as foo.\n"
+    "          enable_bar = enable_foo\n"
+    "        }\n"
+    "\n"
+    "Example\n"
+    "\n"
     "  declare_args() {\n"
     "    enable_teleporter = true\n"
     "    enable_doom_melon = false\n"