ARROW-15388: [C++] Avoid including absl from flatbuffers

There may be an incomplete/broken copy of the Abseil library lying around
(for example because of a race condition while installing it as a bundled library).

Since absl is only used by Flatbuffers to provide a string_view facility,
use our own vendored string_view instead.

Closes #12204 from pitrou/ARROW-15388-flatbuffers-string-view

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/thirdparty/flatbuffers/README.md

@@ -0,0 +1,42 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+This directory contains a vendored version of Flatbuffers
+(unknown changeset), with the following patch for ARROW-15388:
+
+```diff
+diff --git a/cpp/thirdparty/flatbuffers/include/flatbuffers/base.h b/cpp/thirdparty/flatbuffers/include/flatbuffers/base.h
+index 955738067..fccce42f6 100644
+--- a/cpp/thirdparty/flatbuffers/include/flatbuffers/base.h
++++ b/cpp/thirdparty/flatbuffers/include/flatbuffers/base.h
+@@ -212,13 +212,6 @@ namespace flatbuffers {
+         typedef std::experimental::string_view string_view;
+       }
+       #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
+     #endif
+   #endif // __has_include
+ #endif // !FLATBUFFERS_HAS_STRING_VIEW
+```

cpp/thirdparty/flatbuffers/include/flatbuffers/base.h

@@ -212,13 +212,6 @@ namespace flatbuffers {
         typedef std::experimental::string_view string_view;
       }
       #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
     #endif
   #endif // __has_include
 #endif // !FLATBUFFERS_HAS_STRING_VIEW