[libcxx] cmake - guard list remove

[bcain]

Summary

REMOVE_ITEM fails if CMAKE_REQUIRED_LIBRARIES is empty.

[ldionne]

I have a workaround for this locally. I guess this is only relevant with older CMakes.

[bcain]

In D61403#1486958, @ldionne wrote:

I have a workaround for this locally. I guess this is only relevant with older CMakes.

Oh yeah? "Version 3.4.3 " is specified as the minimum on the llvm project docs. Would that one avoid the problem?

I'll push this change but also work on getting our local build system upgraded.

[ldionne]

In D61403#1487077, @bcain wrote:

In D61403#1486958, @ldionne wrote:

I have a workaround for this locally. I guess this is only relevant with older CMakes.

Oh yeah? "Version 3.4.3 " is specified as the minimum on the llvm project docs. Would that one avoid the problem?

I don't know what version of CMake supports that, but I think 3.9.3 doesn't. In any case, the fix is trivial.

[ldionne]

Committed in 9d744bcde993dffe05149be51a9c69178f7a6887

libcxx/cmake/Modules/CheckLibcxxAtomic.cmake

 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_cxx_atomics)
 
 # Perform the check for 64bit atomics without libatomic. It may have been
 # added to the required libraries during in the configuration of LLVM, which
 # would cause the check for CXX atomics without libatomic to incorrectly pass.
+if (CMAKE_REQUIRED_LIBRARIES)
   set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
   list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES "atomic")
   check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
   set(CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES})
+endif()
 
 check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
 # If not, check if the library exists, and atomics work with it.
 if(NOT LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
   if(LIBCXX_HAS_ATOMIC_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
     check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
     if (NOT LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
       message(WARNING "Host compiler must support std::atomic!")
     endif()
   else()
     message(WARNING "Host compiler appears to require libatomic, but cannot find it.")
   endif()
 endif()