[lld][cmake] install symlinks when built standalone

lld installs symlinks with llvm_install_symlink, which relies on the
variable LLVM_USE_SYMLINKS to determine whether to use symlinks or
copies. Since that variable is defined in llvm's CMakeLists.txt, it may
not be defined when lld is built standalone. In that case, default to
using symlinks on Unix platforms. Users can still override this by
setting LLVM_USE_SYMLINKS manually.

Fixes #151557.

Signed-off-by: Ruoyu Zhong <[email protected]>

Author: ZhongRuoyu

lld/cmake/modules/AddLLD.cmake

@@ -73,6 +73,11 @@ macro(add_lld_tool name)
 endmacro()
 
 macro(add_lld_symlink name dest)
+  # In standalone builds, LLVM_USE_SYMLINKS may not be defined.
+  # In that case, default to using symlinks on Unix platforms.
+  if(NOT DEFINED LLVM_USE_SYMLINKS)
+    set(LLVM_USE_SYMLINKS ${CMAKE_HOST_UNIX})
+  endif()
   get_property(LLVM_DRIVER_TOOLS GLOBAL PROPERTY LLVM_DRIVER_TOOLS)
   if(LLVM_TOOL_LLVM_DRIVER_BUILD
      AND ${dest} IN_LIST LLVM_DRIVER_TOOLS