[libc++] Respect Windows Store app CRT restrictions

Some CRT APIs are unavailable for Windows Store apps [1]. Detect when
we're targeting the Windows Store and don't try to refer to non-existent
CRT functions in that case. (This would otherwise lead to a compile
error when using the libc++ headers and compiling for Windows Store.)

[1] https://docs.microsoft.com/en-us/cpp/cppcx/crt-functions-not-supported-in-universal-windows-platform-apps

Differential Revision: https://reviews.llvm.org/D31737

llvm-svn: 299625

libcxx/include/__config

@@ -211,6 +211,16 @@
 # if defined(_LIBCPP_MSVCRT)
 #   define _LIBCPP_HAS_QUICK_EXIT
 # endif
+
+// Some CRT APIs are unavailable to store apps
+#if defined(WINAPI_FAMILY)
+#include <winapifamily.h>
+#if !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP) &&                      \
+    (!defined(WINAPI_PARTITION_SYSTEM) ||                                      \
+     !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_SYSTEM))
+#define _LIBCPP_WINDOWS_STORE_APP
+#endif
+#endif
 #endif // defined(_WIN32)
 
 #ifdef __sun__

libcxx/include/cstdlib

@@ -130,8 +130,10 @@ using ::abort;
 using ::atexit;
 using ::exit;
 using ::_Exit;
+#ifndef _LIBCPP_WINDOWS_STORE_APP
 using ::getenv;
 using ::system;
+#endif
 using ::bsearch;
 using ::qsort;
 using ::abs;