Backport of some build-related flatbuffers changes (#825)

* From flatbuffers upstream: Fix compiler error

Original author of change: avaliente-bc
Backport/update from upstream flatbuffers repository.
Change taken from google/flatbuffers#7227

* From flatbuffers upstream: Fix include of string_view with C++17 abseil

Original author of change: ocpalo
Backport/update from upstream flatbuffers repository.
Changes taken from google/flatbuffers#7897.

Author: ltoenning

include/behaviortree_cpp_v3/flatbuffers/base.h

@@ -237,12 +237,17 @@ namespace flatbuffers {
       }
       #define FLATBUFFERS_HAS_STRING_VIEW 1
     // Check for absl::string_view
-    #elif __has_include("absl/strings/string_view.h")
-      #include "absl/strings/string_view.h"
-      namespace flatbuffers {
-        typedef absl::string_view string_view;
-      }
-      #define FLATBUFFERS_HAS_STRING_VIEW 1
+    #elif __has_include("absl/strings/string_view.h") && \
+          __has_include("absl/base/config.h") && \
+          (__cplusplus >= 201411)
+      #include "absl/base/config.h"
+      #if !defined(ABSL_USES_STD_STRING_VIEW)
+        #include "absl/strings/string_view.h"
+        namespace flatbuffers {
+          typedef absl::string_view string_view;
+        }
+        #define FLATBUFFERS_HAS_STRING_VIEW 1
+      #endif
     #endif
   #endif // __has_include
 #endif // !FLATBUFFERS_HAS_STRING_VIEW

include/behaviortree_cpp_v3/flatbuffers/stl_emulation.h

@@ -625,7 +625,7 @@ class span FLATBUFFERS_FINAL_CLASS {
  private:
   // This is a naive implementation with 'count_' member even if (Extent != dynamic_extent).
   pointer const data_;
-  const size_type count_;
+  size_type count_;
 };
 
  #if !defined(FLATBUFFERS_SPAN_MINIMAL)