Description:
When building the flatbuffer submodule, it fails to finish the build since it wants C++17 features, but only C++14 is enabled during the build.

GCC errors:
```
/usr/include/absl/strings/string_view.h:52:26: error: ‘string_view’ in namespace ‘std’ does not name a type
52 | using string_view = std::string_view;
| ^~~~~~~~~~~
/usr/include/absl/strings/string_view.h:52:21: note: ‘std::string_view’ is only available from C++17 onwards
52 | using string_view = std::string_view;
| ^~~
/usr/include/absl/strings/string_view.h:686:8: error: ‘string_view’ does not name a type
686 | inline string_view ClippedSubstr(string_view s, size_t pos,
| ^~~~~~~~~~~
/usr/include/absl/strings/string_view.h:697:11: error: ‘string_view’ does not name a type
697 | constexpr string_view NullSafeStringView(const char* p) {
```

`-std=gnu++14` is passed to the `/usr/bin/g++-11` command, which prevents the submodule to build.

Additional info:
* package version(s) 1.13.1-2 / git hash 5689e7f44f082ba3c37724c2890e93e7106002a1

The reason for this regression is that the PKGBUILD does not specify git hashes on the submodules and flatbuffer master branch regressed.
A fix would be appending `#commit=<hash>` to known working commits to the urls in the `source` list.


Steps to reproduce:
Run makepkg.