By enabling and disabling optimization flags one by one, I figured out it was '-ipo' (inter-procedural optimization) that was the problem. Without that flag, 'configure' is successful. However, now I have a 'make' error:

./Programs/_testembed.c(1774): error: "__builtin_types_compatible_p" is only allowed in C
          config_set_argv(&config, Py_ARRAY_LENGTH(argv), argv);
                                   ^

compilation aborted for ./Programs/_testembed.c (code 2)
make[3]: *** [Programs/_testembed.o] Error 2