[libc++] Guard fileno() and isatty() usage correctly.

See [1] and [2]. isatty() is part of unistd.h, but fileno() is in
stdio.h, and is guarded by `_POSIX_C_SOURCE >= 1` (on my machine,
`man 3 fileno` only requires `defined(_POSIX_C_SOURCE)`, though).

I ran into this issue while trying to compile libc++ for a bare metal
environment with Newlib as the libc. Newlib defines fileno() in stdio.h,
and its unistd.h does not include stdio.h.

[1]: https://linux.die.net/man/3/fileno
[2]: https://linux.die.net/man/3/isatty

Author: wecing

libcxx/src/print.cpp

@@ -20,8 +20,11 @@
 #  define NOMINMAX
 #  include <io.h>
 #  include <windows.h>
-#elif __has_include(<unistd.h>)
+#elif __has_include(<unistd.h>) && __has_include(<stdio.h>) && \
+      defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 1
+#  include <stdio.h>
 #  include <unistd.h>
+#  define HAS_FILENO_AND_ISATTY
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -56,7 +59,7 @@ __write_to_windows_console([[maybe_unused]] FILE* __stream, [[maybe_unused]] wst
 }
 #  endif // _LIBCPP_HAS_WIDE_CHARACTERS
 
-#elif __has_include(<unistd.h>) // !_LIBCPP_WIN32API
+#elif defined(HAS_FILENO_AND_ISATTY) // !_LIBCPP_WIN32API
 
 _LIBCPP_EXPORTED_FROM_ABI bool __is_posix_terminal(FILE* __stream) { return isatty(fileno(__stream)); }
 #endif