message: build_system: fix Clang warning
  Clang and MSVC are not agreeing on this issue. MSVC complains about an
  implicit capture while Clang says it's an unused lambda capture.
  For now, we'll ignore the Clang warning.
diff: @@ -26,9 +26,9 @@ else()
  set(warnings "${warnings} -Wlogical-op")
  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
  - set(warnings "${warnings} -Wno-missing-braces")
  + set(warnings "${warnings} -Wno-missing-braces -Wno-unused-lambda-capture")
  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
  - set(warnings "${warnings} -Wno-missing-braces -Wno-unused-parameter")
  + set(warnings "${warnings} -Wno-missing-braces -Wno-unused-parameter -Wno-unused-lambda-capture")
  endif()
  # Otherwise tinyxml2 complains.
  