refactor: use FetchContent to download cJSON instead of vendor

GIT_REPOSITORY and GIT_TAG SHALL be changed back if [1] was merged and
upstream released new version.

[1]: DaveGamble/cJSON#949

Signed-off-by: Coelacanthus <uwu@coelacanthus.name>

Author: CoelacanthusHex

CMakeLists.txt

@@ -68,13 +68,45 @@ endif()
 if (NOT DEFINED USE_SYSTEM_DEPS)
     find_package(cJSON)
     if (NOT CJSON_FOUND)
-        add_subdirectory(cjson)
+        if (NOT NO_NETWORK)
+            set(CJSON_OVERRIDE_BUILD_SHARED_LIBS ON)
+            set(CJSON_BUILD_SHARED_LIBS OFF)
+            include(FetchContent)
+            FetchContent_Declare(
+                cJSON
+                # FIXME: change back to DaveGamble/cJSON if upstream fix it
+                # https://github.com/DaveGamble/cJSON/pull/949
+                GIT_REPOSITORY https://github.com/p1k0chu/cJSON
+                GIT_TAG 887642c0a93bd8a6616bf90daacac0ea7d4b095e
+                GIT_PROGRESS ON
+                SOURCE_DIR cjson
+            )
+            FetchContent_MakeAvailable(cJSON)
+            set(CJSON_LIBRARY cJSON)
+        else()
+            message(FATAL_ERROR "System cJSON is not found and network access is not permitted!")
+        endif()
     endif()
 else()
     if (USE_SYSTEM_DEPS)
         find_package(cJSON REQUIRED)
+    elseif(NOT NO_NETWORK)
+        set(CJSON_OVERRIDE_BUILD_SHARED_LIBS ON)
+        set(CJSON_BUILD_SHARED_LIBS OFF)
+        include(FetchContent)
+        FetchContent_Declare(
+            cjson
+            # FIXME: change back to DaveGamble/cJSON if upstream fix it
+            # https://github.com/DaveGamble/cJSON/pull/949
+            GIT_REPOSITORY https://github.com/p1k0chu/cJSON
+            GIT_TAG 887642c0a93bd8a6616bf90daacac0ea7d4b095e
+            GIT_PROGRESS ON
+            SOURCE_DIR cjson
+        )
+        FetchContent_MakeAvailable(cjson)
+        set(CJSON_LIBRARY cjson)
     else()
-        add_subdirectory(cjson)
+        message(FATAL_ERROR "System cJSON is not found and network access is not permitted!")
     endif()
 endif()
 add_subdirectory(cjson-ext)