[clang] Pass option directly to command. NFC

[smeenai]

Summary

This code was written back when LLVM's minimum required CMake version
was 2.8.8, and I assume ExternalProject_Add_Step didn't take this option
at that point. It does now though, so we should just use the option.
Setting the _EP_* property is entirely equivalent (and is in fact how
these commands behave internally), but that also feels like an internal
implementation detail we shouldn't be relying on.

[beanz]

Woo! Thanks for cleaning up one of my old messes :)

clang/CMakeLists.txt

if (CLANG_ENABLE_BOOTSTRAP)
   )
   ExternalProject_Add_StepTargets(${NEXT_CLANG_STAGE} really-install)
   add_custom_target(${NEXT_CLANG_STAGE}-install DEPENDS ${NEXT_CLANG_STAGE}-really-install)
 
   if(NOT CLANG_BOOTSTRAP_TARGETS)
     set(CLANG_BOOTSTRAP_TARGETS check-llvm check-clang check-all)
   endif()
   foreach(target ${CLANG_BOOTSTRAP_TARGETS})
-    # exclude from main target
-    set_target_properties(${NEXT_CLANG_STAGE} PROPERTIES _EP_${target}_EXCLUDE_FROM_MAIN On)
-
     # Install targets have side effects, so we always want to execute them.
     # "install" is reserved by CMake and can't be used as a step name for
     # ExternalProject_Add_Step, so we can match against "^install-" instead of
     # "^install" to get a tighter match. CMake's installation scripts already
     # skip up-to-date files, so there's no behavior change if you install to the
     # same destination multiple times.
     if(target MATCHES "^install-")
       set(step_always ON)
     else()
       set(step_always OFF)
     endif()
 
     ExternalProject_Add_Step(${NEXT_CLANG_STAGE} ${target}
       COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> --target ${target}
       COMMENT "Performing ${target} for '${NEXT_CLANG_STAGE}'"
       DEPENDEES configure
       ALWAYS ${step_always}
+      EXCLUDE_FROM_MAIN ON
       USES_TERMINAL 1
     )
 
     if(target MATCHES "^stage[0-9]*")
       add_custom_target(${target} DEPENDS ${NEXT_CLANG_STAGE}-${target})
     endif()
 
     ExternalProject_Add_StepTargets(${NEXT_CLANG_STAGE} ${target})