added demo target

demo/main.cpp

@@ -0,0 +1,17 @@
+#include <cstdlib>
+#include <iostream>
+#include <string>
+
+int main() {
+  const std::string env_var {"MATADOR_BACKENDS_PATH"};
+
+//  char var[1024];
+//  size_t len{};
+//  const auto error = getenv_s(&len, var, 1024, env_var.c_str());
+//  if (error > 0) {
+//    std::cout << "error: unknown env var " << env_var << "\n";
+//  } else {
+//    std::cout << "env var: " << var << "\n";
+//  }
+  return 0;
+}