Don't vendor nlohmann_json when USE_SYSTEM_DEPS is on.

cmake/icey_build_config.cmake.in

@@ -44,6 +44,9 @@ if(_icey_have_libdatachannel AND DEFINED icey_FIND_COMPONENTS AND "webrtc" IN_LI
 endif()
 
 if(@USE_SYSTEM_DEPS@)
+  if("json" IN_LIST _icey_supported_components)
+    find_dependency(nlohmann_json CONFIG)
+  endif()
   find_package(libuv CONFIG QUIET)
   if(NOT TARGET libuv::uv_a AND NOT TARGET libuv::uv AND NOT TARGET libuv::libuv AND PkgConfig_FOUND)
     pkg_check_modules(LIBUV QUIET IMPORTED_TARGET GLOBAL libuv)

cmake/icey_config.cmake.in

@@ -59,6 +59,9 @@ if(NOT TARGET llhttp_static)
 endif()
 
 if(@USE_SYSTEM_DEPS@)
+  if("json" IN_LIST _icey_supported_components)
+    find_dependency(nlohmann_json CONFIG)
+  endif()
   find_dependency(ZLIB)
   if("archo" IN_LIST _icey_supported_components OR "pacm" IN_LIST _icey_supported_components)
     find_dependency(unofficial-minizip CONFIG QUIET)

icey.cmake

@@ -210,6 +210,9 @@ else()
     pkg_check_modules(LIBUV QUIET IMPORTED_TARGET GLOBAL libuv)
   endif()
   find_package(llhttp CONFIG QUIET)
+  if(BUILD_MODULES AND (NOT DEFINED BUILD_MODULE_json OR BUILD_MODULE_json))
+    find_package(nlohmann_json CONFIG REQUIRED)
+  endif()
   find_package(ZLIB REQUIRED)
   if(NOT TARGET llhttp_static
      AND NOT TARGET llhttp::llhttp_static

src/json/CMakeLists.txt

@@ -1,6 +1,20 @@
-icy_add_module(json
-  DEPENDS base
-  PRETTY_NAME JSON
-)
-# nlohmann/json.hpp is vendored in include/nlohmann/ (MIT licensed, single-header)
-# so no external dependency is needed - icy_add_module already adds our include dir.
+if(USE_SYSTEM_DEPS)
+  icy_add_module(json
+    DEPENDS base
+    PACKAGES nlohmann_json::nlohmann_json
+    PRETTY_NAME JSON
+  )
+else()
+  icy_add_module(json
+    DEPENDS base
+    PRETTY_NAME JSON
+  )
+  target_include_directories(json
+    PUBLIC
+      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/vendor>
+  )
+  install(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/vendor/"
+    DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
+    COMPONENT dev
+  )
+endif()