Fix crashing test executable on gcc 4.9

src/libslic3r/Config.cpp

@@ -271,8 +271,6 @@ ConfigOptionDef* ConfigDef::add_nullable(const t_config_option_key &opt_key, Con
 	return def;
 }
 
-std::string ConfigOptionDef::nocli = "~~~noCLI";
-
 std::ostream& ConfigDef::print_cli_help(std::ostream& out, bool show_defaults, std::function<bool(const ConfigOptionDef &)> filter) const
 {
     // prepare a function for wrapping text

src/libslic3r/Config.hpp

@@ -1444,7 +1444,7 @@ public:
     std::vector<std::string> cli_args(const std::string &key) const;
 
     // Assign this key to cli to disable CLI for this option.
-    static std::string                  nocli;
+    static const constexpr char *nocli =  "~~~noCLI";
 };
 
 // Map from a config option name to its definition.