fixup! cmake: Add HARDENING option

Check `-D_FORTIFY_SOURCE` before applying.

CMakeLists.txt

@@ -521,15 +521,21 @@ if(ENABLE_HARDENING)
     try_append_linker_flag("/HIGHENTROPYVA" TARGET hardening_interface)
     try_append_linker_flag("/NXCOMPAT" TARGET hardening_interface)
   else()
-    # When the build configuration is Debug, all optimizations are disabled.
-    # However, _FORTIFY_SOURCE requires that there is some level of optimization,
-    # otherwise it does nothing and just creates a compiler warning.
-    # Since _FORTIFY_SOURCE is a no-op without optimizations, do not enable it
-    # when the build configuration is Debug.
-    target_compile_options(hardening_interface INTERFACE
-      $<$<NOT:$<CONFIG:Debug>>:-U_FORTIFY_SOURCE>
-      $<$<NOT:$<CONFIG:Debug>>:-D_FORTIFY_SOURCE=3>
+    try_append_cxx_flags("-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3"
+      RESULT_VAR cxx_supports_fortify_source
     )
+    if(cxx_supports_fortify_source)
+      # When the build configuration is Debug, all optimizations are disabled.
+      # However, _FORTIFY_SOURCE requires that there is some level of optimization,
+      # otherwise it does nothing and just creates a compiler warning.
+      # Since _FORTIFY_SOURCE is a no-op without optimizations, do not enable it
+      # when the build configuration is Debug.
+      target_compile_options(hardening_interface INTERFACE
+        $<$<NOT:$<CONFIG:Debug>>:-U_FORTIFY_SOURCE>
+        $<$<NOT:$<CONFIG:Debug>>:-D_FORTIFY_SOURCE=3>
+      )
+    endif()
+    unset(cxx_supports_fortify_source)
 
     try_append_cxx_flags("-Wstack-protector" TARGET hardening_interface SKIP_LINK)
     try_append_cxx_flags("-fstack-protector-all" TARGET hardening_interface)