Add pthread only on Unix systems

The pthread library have being added to Windows build. Which
causes linking problems on Visual Studio.
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

src/CMakeLists.txt

@@ -51,17 +51,17 @@ IF (WIN32)
     SET(LIBS_SYSTEM ws2_32)
 ELSEIF (UNIX)
     IF(CMAKE_SYSTEM_NAME MATCHES "Linux")
-        SET(LIBS_SYSTEM c dl)
+        SET(LIBS_SYSTEM c dl pthread)
     ELSE()
-        SET(LIBS_SYSTEM c)
+        SET(LIBS_SYSTEM c pthread)
     ENDIF()
 ENDIF()
 
 ADD_EXECUTABLE(MQTTVersion MQTTVersion.c)
 ADD_LIBRARY(paho-mqtt3c SHARED ${common_src} MQTTClient.c)
 ADD_LIBRARY(paho-mqtt3a SHARED ${common_src} MQTTAsync.c)
-TARGET_LINK_LIBRARIES(paho-mqtt3c pthread ${LIBS_SYSTEM}) 
-TARGET_LINK_LIBRARIES(paho-mqtt3a pthread ${LIBS_SYSTEM})
+TARGET_LINK_LIBRARIES(paho-mqtt3c ${LIBS_SYSTEM})
+TARGET_LINK_LIBRARIES(paho-mqtt3a ${LIBS_SYSTEM})
 TARGET_LINK_LIBRARIES(MQTTVersion paho-mqtt3a paho-mqtt3c ${LIBS_SYSTEM})
 SET_TARGET_PROPERTIES(
     paho-mqtt3c paho-mqtt3a PROPERTIES
@@ -87,8 +87,8 @@ IF (PAHO_WITH_SSL)
         )
     ADD_LIBRARY(paho-mqtt3cs SHARED ${common_src} MQTTClient.c SSLSocket.c)
     ADD_LIBRARY(paho-mqtt3as SHARED ${common_src} MQTTAsync.c SSLSocket.c)
-    TARGET_LINK_LIBRARIES(paho-mqtt3cs pthread ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
-    TARGET_LINK_LIBRARIES(paho-mqtt3as pthread ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
+    TARGET_LINK_LIBRARIES(paho-mqtt3cs ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
+    TARGET_LINK_LIBRARIES(paho-mqtt3as ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
     SET_TARGET_PROPERTIES(
         paho-mqtt3cs paho-mqtt3as PROPERTIES
         VERSION ${CLIENT_VERSION}