Protect target JsonCpp::JsonCpp against multi-include (#1435)

* Protect target JsonCpp::JsonCpp against multi-include

Fixes #1356

* Simplify (@BillyDonahue)

---------

Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

Author: rjoomen

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 ()