Detect an incomplete libsupc++ (GCC 16 references libstdc++ symbols) by
linking the check with -z defs and a non-elidable new, else use libstdc++.

--- libptytty-2.0.orig/CMakeLists.txt	2026-09-26 21:41:29.366215109 +0200
+++ libptytty-2.0/CMakeLists.txt	2026-09-26 21:41:29.377528241 +0200
@@ -46,11 +46,11 @@ if(${CMAKE_CXX_COMPILER_ID} STREQUAL "GN
     "
      struct e { };
 
-     void f ()
+     e *f ()
      {
        try
          {
-           new e ();
+           return new e ();
          }
        catch (...)
          {
@@ -60,7 +60,7 @@ if(${CMAKE_CXX_COMPILER_ID} STREQUAL "GN
     ")
   execute_process(
     WORKING_DIRECTORY "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp"
-    COMMAND ${CMAKE_C_COMPILER} -x c++ -shared -fPIC -o test.so test.cxx -lsupc++
+    COMMAND ${CMAKE_C_COMPILER} -x c++ -shared -fPIC -Wl,-z,defs -o test.so test.cxx -lsupc++
     OUTPUT_VARIABLE OUTPUT
     ERROR_VARIABLE OUTPUT
     RESULT_VARIABLE RESULT)