--- a/src/ccache/ccache.cpp
+++ b/src/ccache/ccache.cpp
@@ -2181,6 +2181,7 @@ get_manifest_key(Context& ctx, Hash& has
     "OBJCPLUS_INCLUDE_PATH",        // Clang
     "CLANG_CONFIG_FILE_SYSTEM_DIR", // Clang
     "CLANG_CONFIG_FILE_USER_DIR",   // Clang
+    "GCC_HONOUR_COPTS",
     "INCLUDE",                      // MSVC
     "EXTERNAL_INCLUDE",             // MSVC
   };