Add notes that libsystemd-dev or similar is needed.

This is when building with systemd support on Linux.

Closes #2019. Thanks to Mark Symmes.

ChangeLog.txt

@@ -4,6 +4,8 @@ Broker:
 - Fix message expiry interval property not being honoured in
   `mosquitto_broker_publish` and `mosquitto_broker_publish_copy`.
 - Fix websockets listeners with TLS not responding. Closes #2020.
+- Add notes that libsystemd-dev or similar is needed if building with systemd
+  support. Closes #2019.
 
 
 2.0.5 - 2021-01-11

README-compiling.md

@@ -7,6 +7,7 @@ are optional.
 * libwebsockets (optional, disabled by default, version 2.4 and above)
 * cJSON (optional but recommended, for dynamic-security plugin support, and
   JSON output from mosquitto_sub/mosquitto_rr)
+* libsystemd-dev (optional, if building with systemd support on Linux)
 * On Windows, a pthreads library is required if threading support is to be
   included.
 

config.mk

@@ -59,6 +59,8 @@ WITH_SYS_TREE:=yes
 
 # Build with systemd support. If enabled, mosquitto will notify systemd after
 # initialization. See README in service/systemd/ for more information.
+# Setting to yes means the libsystemd-dev or similar package will need to be
+# installed.
 WITH_SYSTEMD:=no
 
 # Build with SRV lookup support.