build now works on nix

1. Static linking - Added KTE_STATIC_LINK CMake option and
   disabled it in default.nix to avoid the "attempted
   static link of dynamic object" error

2. Missing include - Added <cstring> to
   test_swap_edge_cases.cc for std::memset/std::memcpy (GCC
   14 is stricter about transitive includes)

default.nix

@@ -48,6 +48,7 @@ stdenv.mkDerivation {
     "-DBUILD_GUI=${if graphical then "ON" else "OFF"}"
     "-DKTE_USE_QT=${if graphical-qt then "ON" else "OFF"}"
     "-DCMAKE_BUILD_TYPE=Debug"
+    "-DKTE_STATIC_LINK=OFF"
   ];
 
   installPhase = ''