Fix Linux build: default static linking off, add Clang link flag

KTE_STATIC_LINK defaulted to ON, which fails on systems where ncurses
is only available as a shared library. The Nix build already passed
-DKTE_STATIC_LINK=OFF explicitly; this makes the default match.

Also add add_link_options("-stdlib=libc++") for Clang builds — without
it, compilation uses libc++ but the linker defaults to libstdc++,
causing undefined symbol errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -14,7 +14,7 @@ set(BUILD_TESTS ON CACHE BOOL "Enable building test programs.")
 set(KTE_FONT_SIZE "18.0" CACHE STRING "Default font size for GUI")
 option(KTE_UNDO_DEBUG "Enable undo instrumentation logs" OFF)
 option(KTE_ENABLE_TREESITTER "Enable optional Tree-sitter highlighter adapter" OFF)
-option(KTE_STATIC_LINK "Enable static linking on Linux" ON)
+option(KTE_STATIC_LINK "Enable static linking on Linux" OFF)
 
 # Optionally enable AddressSanitizer (ASan)
 option(ENABLE_ASAN "Enable AddressSanitizer for builds" OFF)
@@ -51,6 +51,7 @@ else ()
     )
     if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
         add_compile_options("-stdlib=libc++")
+        add_link_options("-stdlib=libc++")
     else ()
         # nothing special for gcc at the moment
     endif ()