Merge pull request #23 from 5cript/feat/dependency_find

Updated how to find nlohmann.

cmake/dependencies/nlohmann_json.cmake

@@ -1,15 +1,28 @@
+option(ROAR_FIND_NLOHMANN_JSON "Find nlohmann_json first before fetch content" ON)
 option(ROAR_EXTERNAL_NLOHMANN_JSON "Use an external nlohmann_json library (provide it manually)" OFF)
 set(ROAR_NLOHMANN_JSON_GIT_REPOSITORY "https://github.com/nlohmann/json.git" CACHE STRING "The URL from which to clone the nlohmann_json repository")
 set(ROAR_NLOHMANN_JSON_GIT_TAG "8c391e04fe4195d8be862c97f38cfe10e2a3472e" CACHE STRING "The git tag or commit hash to checkout from the nlohmann_json repository")
 
-if (ROAR_EXTERNAL_NLOHMANN_JSON)
+function(roar_fetch_nlohmann_json)
+    if (ROAR_EXTERNAL_NLOHMANN_JSON)
+    else()
+        include(FetchContent)
+        FetchContent_Declare(
+            nlohmann_json
+            GIT_REPOSITORY ${ROAR_NLOHMANN_JSON_GIT_REPOSITORY}
+            GIT_TAG        ${ROAR_NLOHMANN_JSON_GIT_TAG}
+        )
+
+        FetchContent_MakeAvailable(nlohmann_json)
+    endif()
+endfunction()
+
+if (ROAR_FIND_NLOHMANN_JSON)
+    find_package(nlohmann_json)
+
+    if (NOT nlohmann_json_FOUND)
+        roar_fetch_nlohmann_json()
+    endif()
 else()
-    include(FetchContent)
-    FetchContent_Declare(
-        nlohmann_json
-        GIT_REPOSITORY ${ROAR_NLOHMANN_JSON_GIT_REPOSITORY}
-        GIT_TAG        ${ROAR_NLOHMANN_JSON_GIT_TAG}
-    )
-
-    FetchContent_MakeAvailable(nlohmann_json)
+    roar_fetch_nlohmann_json()
 endif()