[libc++] Fix Newlib check in __fwd/ios.h

_NEWLIB_VERSION is only visible when any libc header is included. I ran
into a weird case where during libc++ compilation, __fwd/ios.h did not
see _NEWLIB_VERSION and defined off_t as `long long`, but in the actual
user program, _NEWLIB_VERSION was visible, so the program tried to use a
`long int` instead of `long long` specialization of a template function
that is provided by libc++.a, and caused linking failure.

The new cmake option was also used in another PR that I created; see
#167962.

Author: wecing

libcxx/CMakeLists.txt

@@ -298,6 +298,7 @@ endif()
 
 # Feature options -------------------------------------------------------------
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
+option(LIBCXX_HAS_NEWLIB_LIBC "Build libc++ with support for the Newlib C library" OFF)
 option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
@@ -754,6 +755,7 @@ config_define(${LIBCXX_HAS_PTHREAD_API} _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define(${LIBCXX_HAS_EXTERNAL_THREAD_API} _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define(${LIBCXX_HAS_WIN32_THREAD_API} _LIBCPP_HAS_THREAD_API_WIN32)
 config_define(${LIBCXX_HAS_MUSL_LIBC} _LIBCPP_HAS_MUSL_LIBC)
+config_define(${LIBCXX_HAS_NEWLIB_LIBC} _LIBCPP_HAS_NEWLIB_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define(${LIBCXX_ENABLE_FILESYSTEM} _LIBCPP_HAS_FILESYSTEM)
 config_define(${LIBCXX_ENABLE_RANDOM_DEVICE} _LIBCPP_HAS_RANDOM_DEVICE)

libcxx/include/__config_site.in

@@ -17,6 +17,7 @@
 #cmakedefine01 _LIBCPP_HAS_MONOTONIC_CLOCK
 #cmakedefine01 _LIBCPP_HAS_TERMINAL
 #cmakedefine01 _LIBCPP_HAS_MUSL_LIBC
+#cmakedefine01 _LIBCPP_HAS_NEWLIB_LIBC
 #cmakedefine01 _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine01 _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine01 _LIBCPP_HAS_THREAD_API_WIN32

libcxx/include/__fwd/ios.h

@@ -31,7 +31,7 @@ using wios = basic_ios<wchar_t>;
 template <class _CharT, class _Traits>
 class _LIBCPP_PREFERRED_NAME(ios) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wios)) basic_ios;
 
-#if defined(_NEWLIB_VERSION)
+#if _LIBCPP_HAS_NEWLIB_LIBC
 // On newlib, off_t is 'long int'
 using streamoff = long int; // for char_traits in <string>
 #else