Fix build with gcc version 7.4.0 (#5570)

include/flatbuffers/base.h

@@ -199,7 +199,7 @@ namespace flatbuffers {
   // to detect a header that provides an implementation
   #if defined(__has_include)
     // Check for std::string_view (in c++17)
-    #if __has_include(<string_view>) && (__cplusplus >= 201606 || _HAS_CXX17)
+    #if __has_include(<string_view>) && (__cplusplus >= 201606 || (defined(_HAS_CXX17) && _HAS_CXX17))
       #include <string_view>
       namespace flatbuffers {
         typedef std::string_view string_view;