[libc++] Respect Windows Store app CRT restrictions

[smeenai]

Summary

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

libcxx/trunk/include/__config

 #    define _LIBCPP_MSVCRT // Using Microsoft's C Runtime library
 #  endif
 #  if (defined(_M_AMD64) || defined(__x86_64__)) || (defined(_M_ARM) || defined(__arm__))
 #    define _LIBCPP_HAS_BITSCAN64
 #  endif
 # 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__
 # include <sys/isa_defs.h>
 # ifdef _LITTLE_ENDIAN
 #   define _LIBCPP_LITTLE_ENDIAN 1
 #   define _LIBCPP_BIG_ENDIAN    0
 # else

libcxx/trunk/include/cstdlib

 using ::calloc;
 using ::free;
 using ::malloc;
 using ::realloc;
 using ::abort;
 using ::atexit;
 using ::exit;
 using ::_Exit;
+#ifndef _LIBCPP_WINDOWS_STORE_APP
 using ::getenv;
 using ::system;
+#endif
 using ::bsearch;
 using ::qsort;
 using ::abs;
 using ::labs;
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::llabs;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::div;