Compile warning when compiling in debian unstable

[mochrul]

There are a lot of compile warning about string conversion when compiling the code in debian unstable.

Some example:

proxy.cc: In function 'gboolean z_proxy_policy_config(ZProxy*)':
proxy.cc:420:48: warning: ISO C++ forbids converting a string constant to 'gchar* {aka char*}' [-Wwrite-strings]
   if (!z_proxy_policy_call(self, "config", NULL))
                                                ^
proxy.cc:434:70: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
   PyObject *encryption = z_policy_getattr(self->handler, "encryption");
                                                                      ^
proxy.cc: In function 'gboolean z_proxy_policy_startup(ZProxy*)':
proxy.cc:461:54: warning: ISO C++ forbids converting a string constant to 'gchar* {aka char*}' [-Wwrite-strings]
   if (!z_proxy_policy_call(self, "startup", "startUp"))
                                                      ^
proxy.cc:461:54: warning: ISO C++ forbids converting a string constant to 'gchar* {aka char*}' [-Wwrite-strings]
proxy.cc: In function 'void z_proxy_policy_shutdown(ZProxy*)':
proxy.cc:486:51: warning: ISO C++ forbids converting a string constant to 'gchar* {aka char*}' [-Wwrite-strings]
   z_proxy_policy_call(self, "shutdown", "shutDown");
                                                   ^