Add: [CMake] JSON library (nlohmann)

os/emscripten/cmake/Findnlohmann_json.cmake

@@ -0,0 +1,20 @@
+# nlohmann-json is a custom addition to the emscripten SDK, so it is possible
+# someone patched their SDK. Test out if the SDK supports nlohmann-json.
+include(CheckCXXSourceCompiles)
+set(CMAKE_REQUIRED_FLAGS "-sUSE_NLOHMANN_JSON=1")
+
+check_cxx_source_compiles("
+    #include <nlohmann/json.hpp>
+    int main() { return 0; }"
+    NLOHMANN_JSON_FOUND
+)
+
+if (NLOHMANN_JSON_FOUND)
+        add_library(nlohmann_json INTERFACE IMPORTED)
+        set_target_properties(nlohmann_json PROPERTIES
+                INTERFACE_COMPILE_OPTIONS "-sUSE_NLOHMANN_JSON=1"
+                INTERFACE_LINK_LIBRARIES "-sUSE_NLOHMANN_JSON=1"
+        )
+else()
+        message(WARNING "You are using an emscripten SDK without nlohmann-json support. Please apply 'emsdk-nlohmann_json.patch' to your local emsdk installation.")
+endif()