lld: unify symlink

[martell]

Summary

Not sure if commit r245266 came from D12029 or not but I had a bit more code to go in.
This unifies the CMakeLists.txt to make it closer to the variable names used in llvm-lib.
As well as highlights the reason for the post install script

[ruiu]

LGTM

[martell]

Landed rL246139 and adjusted in rL246140

tools/lld/CMakeLists.txt

 install(TARGETS lld
   RUNTIME DESTINATION bin)
 
-# Create the lld-link[.exe] symlink in the build directory. If symlink is not
-# supported by the operating system, create a copy instead.
 if(UNIX)
-  set(command create_symlink)
-  # Make relative symlink
-  set(src "lld${CMAKE_EXECUTABLE_SUFFIX}")
+  set(LLD_LINK_OR_COPY create_symlink)
+  set(lld_binary "lld${CMAKE_EXECUTABLE_SUFFIX}")
+
 else()
-  set(command copy)
-  set(src "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld${CMAKE_EXECUTABLE_SUFFIX}")
+  set(LLD_LINK_OR_COPY copy)
+  set(lld_binary "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld${CMAKE_EXECUTABLE_SUFFIX}")
 endif()
-set(dst "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX}")
-add_custom_command(TARGET lld POST_BUILD
-  COMMAND ${CMAKE_COMMAND} -E ${command} ${src} ${dst})
+
+set(lld_link "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX}")
+
+add_custom_command(OUTPUT ${lld_link}
+                   COMMAND ${CMAKE_COMMAND} -E ${LLD_LINK_OR_COPY} "${lld_binary}" "${lld_link}"
+                   DEPENDS lld)
 
 install(SCRIPT install_symlink.cmake -DCMAKE_INSTALL_PREFIX=\"${CMAKE_INSTALL_PREFIX}\")

tools/lld/install_symlink.cmake

+# We need to execute this script at installation time because the
+# DESTDIR environment variable may be unset at configuration time.
+# See PR8397.
 if(UNIX)
   set(LINK_OR_COPY create_symlink)
   set(DESTDIR $ENV{DESTDIR})