fix fetching vcpkg

src/gen.cpp

@@ -525,14 +525,14 @@ int generate_cmake(const char *path, bool root) {
     if (!cmake.vcpkg.packages.empty()) {
         cmd("include")("FetchContent");
         cmd("message")("STATUS", "Fetching vcpkg...");
-        cmd("FetchContent_Declare")("vcpkg", "https://github.com/microsoft/vcpkg/archive/refs/tags/2021.05.12.tar.gz");
+        cmd("FetchContent_Declare")("vcpkg", "URL", "https://github.com/microsoft/vcpkg/archive/refs/tags/2021.05.12.tar.gz");
         cmd("FetchContent_MakeAvailable")("vcpkg");
         cmd("include")("${vcpkg_SOURCE_DIR}/scripts/buildsystems/vcpkg.cmake");
         using namespace nlohmann;
         json j;
         j["$schema"] = "https://raw.githubusercontent.com/microsoft/vcpkg/master/scripts/vcpkg.schema.json";
         j["name"] = cmake.project_name;
-        if (!cmake.project_version.empty())
+        if (cmake.project_version.empty())
             throw std::runtime_error("vcpkg manifest mode requires that the project have a version string");
         j["version"] = cmake.project_version;
         j["dependencies"] = cmake.vcpkg.packages;