test: use env var to detect doxygen version

diff --git a/examples/executable/main.cpp b/examples/executable/main.cpp
index 0bf5615..f16246a 100644
--- a/examples/executable/main.cpp
+++ b/examples/executable/main.cpp

@@ -15,6 +15,11 @@
   if (!std::filesystem::exists(out_path + "html")) {
     std::filesystem::create_directory(out_path + "html");
   }
+  // Get the Doxygen version from the environment variable
+  const char* doxygen_version = std::getenv("DEFAULT_DOXYGEN_VERSION");
+  if (!doxygen_version) {
+    doxygen_version = "unknown";
+  }
   // Generate a simple HTML file as documentation
   std::ofstream file(out_path + "html/index.html");
   file << "<!DOCTYPE html>\n"
@@ -31,7 +36,9 @@
           "    <p>Output directory: "
        << out_path
        << "</p>\n"
-          "    <p>Generated by a custom executable mimicking Doxygen 1.15.0.</p>\n"
+          "    <p>Generated by a custom executable mimicking Doxygen "
+       << doxygen_version
+       << ".</p>\n"
           "</body>\n"
           "</html>\n";
   file.close();