Added `CLIENT_STATIC_LDADD` to makefile builds

This allow more libraries to be linked when compiling the clients with a
static libmosquitto, as required for e.g. openssl on some systems.

Closes #1371. Thanks to Fabrice Fontaine.

ChangeLog.txt

@@ -23,6 +23,11 @@ Clients:
 - Fix duplicate cfg definition in rr_client. Closes #1453.
 - Fix `mosquitto_pub -l` hang when stdin stream ends. Closes #1448.
 
+Build:
+- Added `CLIENT_STATIC_LDADD` to makefile builds to allow more libraries to be
+  linked when compiling the clients with a static libmosquitto, as required
+  for e.g. openssl on some systems.
+
 Installer:
 - Fix mosquitto_rr.exe not being included in Windows installers. Closes #1463.
 

client/Makefile

@@ -21,13 +21,13 @@ static : static_pub static_sub static_rr
 	# libmosquitto only.
 
 static_pub : pub_client.o pub_shared.o client_props.o client_shared.o ../lib/libmosquitto.a
-	${CROSS_COMPILE}${CC} $^ -o mosquitto_pub ${CLIENT_LDFLAGS} ${STATIC_LIB_DEPS}
+	${CROSS_COMPILE}${CC} $^ -o mosquitto_pub ${CLIENT_LDFLAGS} ${STATIC_LIB_DEPS} ${CLIENT_STATIC_LDADD}
 
 static_sub : sub_client.o sub_client_output.o client_props.o client_shared.o ../lib/libmosquitto.a
-	${CROSS_COMPILE}${CC} $^ -o mosquitto_sub ${CLIENT_LDFLAGS} ${STATIC_LIB_DEPS}
+	${CROSS_COMPILE}${CC} $^ -o mosquitto_sub ${CLIENT_LDFLAGS} ${STATIC_LIB_DEPS} ${CLIENT_STATIC_LDADD}
 
 static_rr : rr_client.o client_props.o client_shared.o pub_shared.o sub_client_output.o ../lib/libmosquitto.a
-	${CROSS_COMPILE}${CC} $^ -o mosquitto_rr ${CLIENT_LDFLAGS} ${STATIC_LIB_DEPS}
+	${CROSS_COMPILE}${CC} $^ -o mosquitto_rr ${CLIENT_LDFLAGS} ${STATIC_LIB_DEPS} ${CLIENT_STATIC_LDADD}
 
 mosquitto_pub : pub_client.o pub_shared.o client_shared.o client_props.o
 	${CROSS_COMPILE}${CC} $(CLIENT_LDFLAGS) $^ -o $@ $(CLIENT_LDADD)

config.mk

@@ -82,6 +82,11 @@ WITH_STRIP:=no
 # Build static libraries
 WITH_STATIC_LIBRARIES:=no
 
+# Use this variable to add extra library dependencies when building the clients
+# with the static libmosquitto library. This may be required on some systems
+# where e.g. -lz or -latomic are needed for openssl.
+CLIENT_STATIC_LDADD:=
+
 # Build shared libraries
 WITH_SHARED_LIBRARIES:=yes