[C++] Updates real_path to be truly portable (#5787)

* Removes posix definition

stat and fseeko are not used. Tested on QNX (GCC 5.4.0) and MinGw

* Updates realpath to more modern (2008) version

* Removes unix specific headers

* Adds detection of strtoll_l as android API < 21 does not provide it

* Includes cstdlib and formatting

Removes cmakelist alterations as not necessary. Formatting not complete.

* Stdlib outside if

Stdlib is available on all platforms

* Fixes indentation

* Adds locale check to android build

* Adds missing brace

* full names

* Removes again, no clue

* Updates base to check for locale independent android

Cmake already checks for others, also MSC?

* Changes to test on available and not requested

* Fixes android bad xopen_version define

* Removes warning

include/flatbuffers/base.h

@@ -46,6 +46,10 @@
 #include <iterator>
 #include <memory>
 
+#if defined(__unix__) && !defined(FLATBUFFERS_LOCALE_INDEPENDENT)
+#include <unistd.h>
+#endif 
+
 #ifdef _STLPORT_VERSION
   #define FLATBUFFERS_CPP98_STL
 #endif
@@ -53,6 +57,10 @@
   #include <functional>
 #endif
 
+#ifdef __ANDROID__
+  #include <android/api-level.h>
+#endif
+
 #include "flatbuffers/stl_emulation.h"
 
 #if defined(__ICCARM__)
@@ -236,10 +244,8 @@ namespace flatbuffers {
 
 #ifndef FLATBUFFERS_LOCALE_INDEPENDENT
   // Enable locale independent functions {strtof_l, strtod_l,strtoll_l, strtoull_l}.
-  // They are part of the POSIX-2008 but not part of the C/C++ standard.
-  // GCC/Clang have definition (_XOPEN_SOURCE>=700) if POSIX-2008.
   #if ((defined(_MSC_VER) && _MSC_VER >= 1800)            || \
-       (defined(_XOPEN_SOURCE) && (_XOPEN_SOURCE>=700)))
+       (defined(_XOPEN_VERSION) && (_XOPEN_VERSION>=700)) && (!defined(__ANDROID_API__) || (defined(__ANDROID_API__) && (__ANDROID_API__>=21))))
     #define FLATBUFFERS_LOCALE_INDEPENDENT 1
   #else
     #define FLATBUFFERS_LOCALE_INDEPENDENT 0