如果遇到报错
ADD_LIBRARY cannot create target "paho-mqtt3c" because another target with the same name already exists. The existing target is a shared library created in source directory
需要更改
..\paho.mqtt.c\src
下的CMakeLists.txt，将 -static后缀添加到文件中的静态库名称上，有两个地方需要进行此更改，在该文件中搜索PAHO_BUILD_STATIC：
第94行附近（添加-static到if语句中引用的库名称中）：
IF (PAHO_BUILD_STATIC)
ADD_LIBRARY(paho-mqtt3c-static STATIC $<TARGET_OBJECTS:common_obj> MQTTClient.c)
ADD_LIBRARY(paho-mqtt3a-static STATIC $<TARGET_OBJECTS:common_obj> MQTTAsync.c)
TARGET_LINK_LIBRARIES(paho-mqtt3c-static ${LIBS_SYSTEM})
TARGET_LINK_LIBRARIES(paho-mqtt3a-static ${LIBS_SYSTEM})
INSTALL(TARGETS paho-mqtt3c-static paho-mqtt3a-static
ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
ENDIF()
在第164行附近（添加-static到if语句中引用的库名称中）：
IF (PAHO_BUILD_STATIC)
ADD_LIBRARY(paho-mqtt3cs-static STATIC $<TARGET_OBJECTS:common_ssl_obj> MQTTClient.c SSLSocket.c)
ADD_LIBRARY(paho-mqtt3as-static STATIC $<TARGET_OBJECTS:common_ssl_obj> MQTTAsync.c SSLSocket.c)
TARGET_LINK_LIBRARIES(paho-mqtt3cs-static ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
TARGET_LINK_LIBRARIES(paho-mqtt3as-static ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
SET_TARGET_PROPERTIES(
paho-mqtt3cs-static paho-mqtt3as-static PROPERTIES
VERSION ${CLIENT_VERSION}
SOVERSION ${PAHO_VERSION_MAJOR}
COMPILE_DEFINITIONS "OPENSSL=1")
INSTALL(TARGETS paho-mqtt3cs-static paho-mqtt3as-static
ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
ENDIF()
进行更改后，清除您的CMake缓存（或删除构建文件夹中的CMakeCache.txt文件）并重新运行CMake。