[libc++] Don't alias quick_exit if __ANDROID_API__ < 21

[thomasanderson]

Summary

quick_exit() and at_quick_exit() were introduced in android NDK 21:
https://android.googlesource.com/platform/prebuilts/ndk/+/dev/platform/sysroot/usr/include/stdlib.h#55

This CL conditions _LIBCPP_HAS_QUICK_EXIT on __ANDROID_API__ >= 21. The only place this macro is used is in some using declarations: using ::quick_exit, using ::at_quick_exit.

Also, add a missing include to sys/cdefs.h which is what defines __BIONIC__.

[danalbert]

I'm wondering how you came across this as an issue. libc++ with the NDK needs to be accompanied by libandroid_support, which provides a decl (though not a definition) for this. I prefer this approach, but was under the impression that all of the hacks for "this wasn't in bionic until three years ago, and that still matters" weren't wanted upstream.

include/__config

301 ↗

(On Diff #121066)

This should maybe be a __has_include, since bionic isn't only for Android these days.

libcxx/trunk/include/__config

 #endif  // !defined(_LIBCPP_LITTLE_ENDIAN) && !defined(_LIBCPP_BIG_ENDIAN)
 
 #if __has_attribute(__no_sanitize__) && !defined(_LIBCPP_COMPILER_GCC)
 #define _LIBCPP_NO_CFI __attribute__((__no_sanitize__("cfi")))
 #else
 #define _LIBCPP_NO_CFI
 #endif
 
+#if __libcpp_has_include(<sys/cdefs.h>)
+#include <sys/cdefs.h>
+#endif
+
 #if defined(_LIBCPP_COMPILER_CLANG)
 
 // _LIBCPP_ALTERNATE_STRING_LAYOUT is an old name for
 // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT left here for backward compatibility.
 #if (defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&       \
      (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)) ||                           \
     defined(_LIBCPP_ALTERNATE_STRING_LAYOUT)
 #define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT

 #if defined(__FreeBSD__)
 #define _LIBCPP_HAS_QUICK_EXIT
 #define _LIBCPP_HAS_C11_FEATURES
 #elif defined(__Fuchsia__)
 #define _LIBCPP_HAS_QUICK_EXIT
 #define _LIBCPP_HAS_C11_FEATURES
 #elif defined(__linux__)
 #if !defined(_LIBCPP_HAS_MUSL_LIBC)
-#if __GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
+#if __GLIBC_PREREQ(2, 15) || (defined(__BIONIC__) && (__ANDROID_API__ >= 21))
 #define _LIBCPP_HAS_QUICK_EXIT
 #endif
 #if __GLIBC_PREREQ(2, 17)
 #define _LIBCPP_HAS_C11_FEATURES
 #endif
 #else // defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_HAS_QUICK_EXIT
 #define _LIBCPP_HAS_C11_FEATURES