mosquitto 2.0.20 fails to build on Linux without threads

[jacmet]

As reported on the mailing list (https://www.eclipse.org/lists/mosquitto-dev/msg03003.html), the changes in 88b7bb3 unfortunately broke the builds on Linux on setups without threads, E.G.:

make ... WITH_THREADING=no WITH_TLS=yes WITH_CJSON=no WITH_SRV=no WITH_WEBSOCKETS=no
make[1]: Entering directory '/home/autobuild/autobuild/instance-7/output-1/build/mosquitto-2.0.20'
set -e; for d in lib client; do /usr/bin/make -C ${d}; done
make[2]: Entering directory '/home/autobuild/autobuild/instance-7/output-1/build/mosquitto-2.0.20/lib'
/home/autobuild/autobuild/instance-7/output-1/per-package/mosquitto/host/bin/riscv64-buildroot-linux-uclibc-gcc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I. -I.. -I../include -I../../include -DWITH_TLS -DOPENSSL_API_COMPAT=0x10100000L -DWITH_TLS_PSK -DWITH_SOCKS -DWITH_UNIX_SOCKETS -I../deps -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Ofast -g0  -fPIC  -Wl,-elf2flt=-r -static -fPIC -c mosquitto.c -o mosquitto.o
/home/autobuild/autobuild/instance-7/output-1/per-package/mosquitto/host/bin/riscv64-buildroot-linux-uclibc-gcc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I. -I.. -I../include -I../../include -DWITH_TLS -DOPENSSL_API_COMPAT=0x10100000L -DWITH_TLS_PSK -DWITH_SOCKS -DWITH_UNIX_SOCKETS -I../deps -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Ofast -g0  -fPIC  -Wl,-elf2flt=-r -static -fPIC -c actions.c -o actions.o
actions.c: In function 'mosquitto_publish_v5':
actions.c:158:51: error: 'struct mosquitto_msg_data' has no member named 'mutex'
  158 |                 pthread_mutex_lock(&mosq->msgs_out.mutex);
      |                                                   ^
actions.c:161:53: error: 'struct mosquitto_msg_data' has no member named 'mutex'
  161 |                 pthread_mutex_unlock(&mosq->msgs_out.mutex);
      |                                                     ^

I see you replaced most, but not all pthread_* calls by COMPAT_pthread_*? Was this done on purpose or just a mistake?

[OgreTransporter]

For mosquitto, threads are now standard. They are simply used in many places without verification: #3193

[ralight]

This is fixed in the fixes branch and will be part of 2.0.21.