avoid picking up abseil-cpp if found

https://trac.osgeo.org/postgis/ticket/5178
https://github.com/MapServer/MapServer/pull/6833/
Index: deps/flatgeobuf/include/flatbuffers/base.h
--- deps/flatgeobuf/include/flatbuffers/base.h.orig
+++ deps/flatgeobuf/include/flatbuffers/base.h
@@ -233,7 +233,7 @@ namespace flatbuffers {
       }
       #define FLATBUFFERS_HAS_STRING_VIEW 1
     // Check for absl::string_view
-    #elif __has_include("absl/strings/string_view.h")
+    #elif __has_include("absl/strings/string_view.h") && (__cplusplus >= 201411)
       #include "absl/strings/string_view.h"
       namespace flatbuffers {
         typedef absl::string_view string_view;
