Bug with multithreading

[OgreTransporter]

If you try to compile mosquitto without multithreading (-DWITH_THREADING=OFF) you will get a number of error messages, because mosquitto_internal.h line 271 checks whether threading is enabled or not, but not in the rest of the library.

mosquitto/lib/mosquitto_internal.h

Line 271 in 849e0f5

#if defined(WITH_THREADING) && !defined(WITH_BROKER)

For example, an attempt is made in handle_connack.c line 120 to use the variable state_mutex, but this is not defined if no threads are activated.

mosquitto/lib/handle_connack.c

Line 120 in 849e0f5

pthread_mutex_lock(&mosq->state_mutex);

pthread is now used without verification in so many files that the library must be assumed to be required.

[ralight]

Thanks for the report, but closing because it's a duplicate of #3183. This is anyway fixed for 2.0.21.