The iOS product type 'com.apple.package-type.bundle.unit-test' requires
code signing on Xcode 6. Other iOS target types do too. Until CMake
learns to add the CODE_SIGN_IDENTITY build attribute itself, toolchain
files can set CMAKE_XCODE_ATTRIBUTE_CODE_SIGN_IDENTITY to tell the Xcode
generator to add the attribute. Teach CMakeDetermineCompilerId to
recognize this variable and add the CODE_SIGN_IDENTITY build attribute
to the compiler id project.
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