diff --git a/llvm/cmake/modules/LLVMInstallSymlink.cmake b/llvm/cmake/modules/LLVMInstallSymlink.cmake
--- a/llvm/cmake/modules/LLVMInstallSymlink.cmake
+++ b/llvm/cmake/modules/LLVMInstallSymlink.cmake
@@ -4,9 +4,13 @@
 
 include(GNUInstallDirs)
 
+set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/../../../cmake/Modules" ${CMAKE_MODULE_PATH})
+include(ExtendPath)
+
 function(install_symlink name target outdir)
   set(DESTDIR $ENV{DESTDIR})
-  set(bindir "${DESTDIR}${CMAKE_INSTALL_PREFIX}/${outdir}")
+  extend_path(prefixed_outdir "${CMAKE_INSTALL_PREFIX}" "${outdir}")
+  set(bindir "${DESTDIR}${prefixed_outdir}")
 
   message(STATUS "Creating ${name}")