Fix build for platforms not supporting realpath

Added a check for a preprocessor definition that can be set if the platform you're building for doesn't support any notion of absolute path resolution/realpath()/etc.

include/flatbuffers/util.h

@@ -219,6 +219,9 @@ inline void EnsureDirExists(const std::string &filepath) {
 // Obtains the absolute path from any other path.
 // Returns the input path if the absolute path couldn't be resolved.
 inline std::string AbsolutePath(const std::string &filepath) {
+#ifdef NO_ABSOLUTE_PATH_RESOLUTION
+	return filepath;
+#else
   #ifdef _WIN32
     char abs_path[MAX_PATH];
     return GetFullPathNameA(filepath.c_str(), MAX_PATH, abs_path, nullptr)
@@ -228,6 +231,7 @@ inline std::string AbsolutePath(const std::string &filepath) {
   #endif
     ? abs_path
     : filepath;
+#endif // NO_ABSOLUTE_PATH_RESOLUTION
 }
 
 // To and from UTF-8 unicode conversion functions