C11 functions quick_exit and at_quick_exit should be visible in C++11.
This is a follow-up to #9545.
Although C++11 defines STDC_VERSION to C99, it includes two functions from C11: quick_exit and at_quick_exit.
In iso/stdlib_c11.h the conditional for these two functions should look like:
#if !defined(_STRICT_SYMBOLS) || defined(_STDC_C11) || __cplusplus >= 201103L
extern int at_quick_exit(void ()(void));
extern _NORETURN_KYWD void quick_exit(int);
#endif / !_STRICT_SYMBOLS || _STDC_C11 || __cplusplus >= 201103L */
while no modification is required for aligned_alloc.