Mute -Wdeprecated-declarations error when thread sanitizer enabled on darwin platform

При сборке проектов с ```--sanitize=thread``` под **darwin**, по умолчанию попадет ```util``` и его часть ```system/context.h```, где в случае ```thread_sanitizer_enabled``` выставляется ```USE_UCONTEXT_CONT```, что приводит к использованию [депрекейтнутного](https://opensource.apple.com/source/libplatform/libplatform-125/include/ucontext.h.auto.html) ```<ucontext.h>``` из [macos_sdk](https://a.yandex-team.ru/arcadia/build/platform/macos_sdk/ya.make?rev=rXXXXXX#L6).


Без должных изменений, сборка падает с ошибкой.
```In file included from /Users/hiddenpath/.ya/build/build_root/fnjm/000128/util/all_system_1.cpp:13:
In file included from /Users/hiddenpath/arcadia/util/system/context.cpp:31:
In file included from /Users/hiddenpath/arcadia/util/system/context.h:46:
/Users/hiddenpath/.ya/tools/v4/2088833948/usr/include/ucontext.h:51:2: error: The deprecated ucontext routines require _XOPEN_SOURCE to be defined
#error The deprecated ucontext routines require _XOPEN_SOURCE to be defined
 ^
In file included from /Users/hiddenpath/.ya/build/build_root/fnjm/000128/util/all_system_1.cpp:13:
In file included from /Users/hiddenpath/arcadia/util/system/context.cpp:31:
/Users/hiddenpath/arcadia/util/system/context.h:68:9: error: use of undeclared identifier 'getcontext'
        getcontext(&Ctx_);
        ^
/Users/hiddenpath/arcadia/util/system/context.h:72:9: error: use of undeclared identifier 'getcontext'
        getcontext(&Ctx_);
        ^
/Users/hiddenpath/arcadia/util/system/context.h:80:9: error: use of undeclared identifier 'makecontext'
        makecontext(&Ctx_, (ucontext_func_t)ContextTrampoLine, 1, c.TrampoLine);
        ^
/Users/hiddenpath/arcadia/util/system/context.h:87:9: error: use of undeclared identifier 'swapcontext'
        swapcontext(&Ctx_, &next->Ctx_);
        ^
5 errors generated.
Failed

1 files changed, 10 insertions, 0 deletions

diff --git a/util/system/context.h b/util/system/context.h
index 91c0831338..c04a462365 100644
--- a/util/system/context.h
+++ b/util/system/context.h
@@ -22,6 +22,11 @@
  */
 #if defined(thread_sanitizer_enabled)
     #define USE_UCONTEXT_CONT
+    #if defined(_darwin_)
+        #define _XOPEN_SOURCE 700
+        #pragma clang diagnostic push
+        #pragma clang diagnostic ignored "-Wdeprecated-declarations"
+    #endif
 #elif defined(_bionic_) || defined(__IOS__)
     #define USE_GENERIC_CONT
 #elif defined(_cygwin_)
@@ -172,6 +177,11 @@ static inline size_t MachineContextSize() noexcept {
     #undef EXTRA_PUSH_ARGS
 #endif
 
+#if defined(_darwin_) && defined(thread_sanitizer_enabled)
+    #pragma clang diagnostic pop
+    #undef _XOPEN_SOURCE
+#endif
+
 struct TExceptionSafeContext: public TContMachineContext {
     using TContMachineContext::TContMachineContext;