【 解决jsoncpp被多重引入问题add_library cannot create imported target “JsonCpp::JsonCpp“】

项目中需要使用jsoncpp来解析json文件，但是当前代码里面的依赖package也有包含jsocpp，导致jsoncpp被多次引入，造成编译错误：如下：
CMake Error at /home/a/work/install/jsoncpp/lib/cmake/jsoncpp/jsoncpp-namespaced-targets.cmake:5 (add_library):
add_library cannot create imported target “JsonCpp::JsonCpp” because
another target with the same name already exists.
Call Stack (most recent call first):
/home/a/work/zeekros/install/jsoncpp/lib/cmake/jsoncpp/jsoncppConfig.cmake:31 (include)
/usr/share/cmake-3.16/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
CMakeLists.txt:62 (find_package)

解决方案：
修改jsoncpp仓库的jsoncpp-namespaced-targets.cmake 里面加上TARGET JsonCpp::JsonCpp检查就可以了

if (TARGET jsoncpp_static AND NOT TARGET JsonCpp::JsonCpp)
    add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
    set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES "jsoncpp_static")
elseif (TARGET jsoncpp_lib AND NOT TARGET JsonCpp::JsonCpp)
    add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
    set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES "jsoncpp_lib")
endif ()

解决方案参考
https://github.com/open-source-parsers/jsoncpp/issues/1356

This is happening because the jsoncpp-namespaced-targets.cmake file is not checking if the target exists. A simple change adding a check to see if JsonCPP::JsonCPP is defined should fix this problem:

if (TARGET jsoncpp_static AND NOT TARGET JsonCpp::JsonCpp)
add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES “jsoncpp_static”)
elseif (TARGET jsoncpp_lib AND NOT TARGET JsonCpp::JsonCpp)
add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES “jsoncpp_lib”)
endif ()