fix CMake and nlohmann_json

Author: fresh-borzoni

bindings/cpp/CMakeLists.txt

@@ -255,14 +255,17 @@ if (FLUSS_ENABLE_TESTING)
     )
     set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
 
-    set(JSON_BuildTests OFF CACHE INTERNAL "")
-    FetchContent_Declare(
-        nlohmann_json
-        URL https://github.com/nlohmann/json/archive/refs/tags/v${FLUSS_NLOHMANN_JSON_VERSION}.tar.gz
-        URL_HASH SHA256=0d8ef5af7f9794e3263480193c491549b2ba6cc74bb018906202ada498a79406
-    )
-
-    FetchContent_MakeAvailable(googletest nlohmann_json)
+    FetchContent_MakeAvailable(googletest)
+
+    if (NOT TARGET nlohmann_json::nlohmann_json)
+        set(JSON_BuildTests OFF CACHE INTERNAL "")
+        FetchContent_Declare(
+            nlohmann_json
+            URL https://github.com/nlohmann/json/archive/refs/tags/v${FLUSS_NLOHMANN_JSON_VERSION}.tar.gz
+            URL_HASH SHA256=0d8ef5af7f9794e3263480193c491549b2ba6cc74bb018906202ada498a79406
+        )
+        FetchContent_MakeAvailable(nlohmann_json)
+    endif()
 
     enable_testing()
     include(GoogleTest)