cmake export configuration: allow repeating find_package(jsoncpp) calls

In jsoncpp-namspaced-targets.cmake, it creates JsonCpp::JsonCpp imported
library without first checking whether it was already created by former
call to find_package(JsonCpp).  As CMake allows repeated call to
find_package(), the error of "another target with the same name already
exists" should be fixed.

jsoncpp-namespaced-targets.cmake

@@ -1,7 +1,9 @@
-if (TARGET jsoncpp_static)
-    add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
-    set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES "jsoncpp_static")
-elseif (TARGET jsoncpp_lib)
-    add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
-    set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES "jsoncpp_lib")
-endif ()
+if (NOT TARGET JsonCpp::JsonCpp)
+    if (TARGET jsoncpp_static)
+        add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
+        set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES "jsoncpp_static")
+    elseif (TARGET jsoncpp_lib)
+        add_library(JsonCpp::JsonCpp INTERFACE IMPORTED)
+        set_target_properties(JsonCpp::JsonCpp PROPERTIES INTERFACE_LINK_LIBRARIES "jsoncpp_lib")
+    endif ()
+endif ()