Don't try to FetchContent nlohmann-json if it is already installed. R…

…elated to #105

CMakeLists.txt

@@ -102,15 +102,18 @@ find_package(PCL REQUIRED COMPONENTS common io octree filters features geometry
 #
 # Get the nlohmann/json package
 #
-# Use the next line to get it through normal means
-# find_package(nlohmann_json 3.2.0 REQUIRED)
-# Or use these lines to fetch it as needed.
+# First see if it is already installed.
 
-set(FETCHCONTENT_QUIET OFF)
-include(FetchContent)
+find_package(nlohmann_json 3.11.2)
 
-FetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz)
-FetchContent_MakeAvailable(json)
+if(NOT nlohmann_json_FOUND)
+	# If it is not installed already we download a private copy.
+	set(FETCHCONTENT_QUIET OFF)
+	include(FetchContent)
+
+	FetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz)
+	FetchContent_MakeAvailable(json)
+endif()
 
 #
 # Optional packages, required by some submodules