cxxopts: Unpin icu, fix build

Set -DCXXOPTS_CXX_STANDARD=17 while building to avoid icu-related error.

This is supported by cxxopts and included in their testing [1]. The
build just *defaults* to C++11, which icu >= 75 no longer supports.

[1]: https://github.com/jarro2783/cxxopts/commit/76bd60dc17ca514f2febb49382b38d526d5502a5

pkgs/by-name/cx/cxxopts/package.nix

@@ -4,7 +4,7 @@
   fetchFromGitHub,
   fetchpatch2,
   cmake,
-  icu74,
+  icu,
   pkg-config,
   testers,
   validatePkgConfig,
@@ -22,9 +22,10 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-baM6EX9D0yfrKxuPXyUUV9RqdrVLyygeG6x57xN8lc4=";
   };
 
-  propagatedBuildInputs = lib.optionals enableUnicodeHelp [ icu74.dev ];
+  propagatedBuildInputs = lib.optionals enableUnicodeHelp [ icu.dev ];
   cmakeFlags = [
     "-DCXXOPTS_BUILD_EXAMPLES=OFF"
+    "-DCXXOPTS_CXX_STANDARD=17"
   ]
   ++ lib.optional enableUnicodeHelp "-DCXXOPTS_USE_UNICODE_HELP=TRUE";
   nativeBuildInputs = [