Bump C++ requirement to C++17

The 1.13.0 release of Poco requires a newer version of C++, specifically
C++17. The configuration phase will fail to find the Poco headers with
an older C++ version.

Changes to ./configure may be reproduced by running autoconf v2.72.

Changelog: https://raw.githubusercontent.com/pocoproject/poco/poco-1.13.0-release/CHANGELOG
Nixpkgs patch: https://github.com/NixOS/nixpkgs/blob/6dd5612cdd562163dc7a7341e3be9c3f6f4d9078/pkgs/tools/graphics/sanjuuni/default.nix#L22-L26
Relevant discussion: NixOS/nixpkgs#283809 (comment)

README.md

@@ -44,7 +44,7 @@ Nix support is maintained by [Tomodachi94](https://github.com/tomodachi94). For
 
 ## Building
 Requirements:
-* C++11 or later compiler
+* C++17 or later compiler
 * FFmpeg libraries
 * Poco (Foundation and Util required; Net/NetSSL optional)
 * zlib (usually required by Poco)