[libc++] Add configuration define for off_t functions

[smeenai]

Summary

Create this define in __config and use it elsewhere, instead of checking
the operating system/library defines in other files. The aim is to
reduce the usage of _WIN32 outside __config. No functional change.

[mclow.lists]

I like this; I would like to see no mentions of _WIN32 outside of <__config>.
That being said, we usually write things in the negative in libc++.

So the flag would be _LIBCPP_HAS_NO_OFF_T_FUNCTIONS, and the tests would be #ifdef _LIBCPP_HAS_NO_OFF_T_FUNCTIONS

[smeenai]

Addressing @mclow.lists's comment

libcxx/trunk/include/__config

 #else
 #define _LIBCPP_SAFE_STATIC
 #endif
 
 #if !__has_builtin(__builtin_addressof) && _GNUC_VER < 700
 #define _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
 #endif
 
+#if !defined(_LIBCPP_HAS_NO_OFF_T_FUNCTIONS)
+#if defined(_WIN32) || defined(_NEWLIB_VERSION)
+#define _LIBCPP_HAS_NO_OFF_T_FUNCTIONS
+#endif
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

libcxx/trunk/include/fstream

case ios_base::cur:
         __whence = SEEK_CUR;
         break;
     case ios_base::end:
         __whence = SEEK_END;
         break;
     default:
         return pos_type(off_type(-1));
     }
-#if defined(_WIN32) || defined(_NEWLIB_VERSION)
+#if defined(_LIBCPP_HAS_NO_OFF_T_FUNCTIONS)
     if (fseek(__file_, __width > 0 ? __width * __off : 0, __whence))
         return pos_type(off_type(-1));
     pos_type __r = ftell(__file_);
 #else
     if (fseeko(__file_, __width > 0 ? __width * __off : 0, __whence))
         return pos_type(off_type(-1));
     pos_type __r = ftello(__file_);
 #endif
     __r.state(__st_);
     return __r;
 }
 
 template <class _CharT, class _Traits>
 typename basic_filebuf<_CharT, _Traits>::pos_type
 basic_filebuf<_CharT, _Traits>::seekpos(pos_type __sp, ios_base::openmode)
 {
     if (__file_ == 0 || sync())
         return pos_type(off_type(-1));
-#if defined(_WIN32) || defined(_NEWLIB_VERSION)
+#if defined(_LIBCPP_HAS_NO_OFF_T_FUNCTIONS)
     if (fseek(__file_, __sp, SEEK_SET))
         return pos_type(off_type(-1));
 #else
     if (fseeko(__file_, __sp, SEEK_SET))
         return pos_type(off_type(-1));
 #endif
     __st_ = __sp.state();
     return __sp;
else if (__cm_ & ios_base::in)
                     const int __off =  __cv_->length(__state, __extbuf_,
                                                      __extbufnext_,
                                                      this->gptr() - this->eback());
                     __c += __extbufnext_ - __extbuf_ - __off;
                     __update_st = true;
                 }
             }
         }
-#if defined(_WIN32) || defined(_NEWLIB_VERSION)
+#if defined(_LIBCPP_HAS_NO_OFF_T_FUNCTIONS)
         if (fseek(__file_, -__c, SEEK_CUR))
             return -1;
 #else
         if (fseeko(__file_, -__c, SEEK_CUR))
             return -1;
 #endif
         if (__update_st)
             __st_ = __state;