[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

src/util.cpp

@@ -16,7 +16,10 @@
 
 // clang-format off
 // Dont't remove `format off`, it prevent reordering of win-includes.
-#define _POSIX_C_SOURCE 200112L // For stat from stat/stat.h and fseeko() (POSIX extensions).
+
+#  define _POSIX_C_SOURCE 200809L
+#  define _XOPEN_SOURCE 700L
+
 #ifdef _WIN32
 #  ifndef WIN32_LEAN_AND_MEAN
 #    define WIN32_LEAN_AND_MEAN
@@ -31,9 +34,6 @@
 #  include <direct.h>
 #  include <winbase.h>
 #  undef interface  // This is also important because of reasons
-#else
-#  define _XOPEN_SOURCE 600 // For PATH_MAX from limits.h (SUSv2 extension) 
-#  include <limits.h>
 #endif
 // clang-format on
 
@@ -42,6 +42,7 @@
 
 #include <sys/stat.h>
 #include <clocale>
+#include <cstdlib>
 #include <fstream>
 
 namespace flatbuffers {
@@ -196,8 +197,14 @@ std::string AbsolutePath(const std::string &filepath) {
       char abs_path[MAX_PATH];
       return GetFullPathNameA(filepath.c_str(), MAX_PATH, abs_path, nullptr)
     #else
-      char abs_path[PATH_MAX];
-      return realpath(filepath.c_str(), abs_path)
+      char *abs_path_temp = realpath(filepath.c_str(), nullptr);
+      bool success = abs_path_temp != nullptr;
+      std::string abs_path;
+      if(success) {
+        abs_path = abs_path_temp;
+        free(abs_path_temp);
+      }
+      return success
     #endif
       ? abs_path
       : filepath;