There is more

In addition to CMAKE_HOST_SYSTEM_PROCESSOR, CMake also defines the CMAKE_SYSTEM_PROCESSOR variable. Whereas the former contains the name of the CPU CMake is currently running on, the latter will contain the name of the CPU we are currently building for. This is a subtle difference that plays a very fundamental role when cross-compiling. We will see more about cross-compilation in Chapter 13, Alternative Generators and Cross-compilation.

An alternative to letting CMake detect the host processor architecture is to use symbols defined within C or C++ and use CMake's try_run function to build and attempt to execute the source code (see Chapter 5, Configure-time and Build-time Operations, Recipe 8, Probing execution) that is branched by the preprocessor symbols. This returns well-defined errors that can be caught on the CMake side (this strategy is inspired by https://github.com/axr/solar-cmake/blob/master/TargetArch.cmake):

#if defined(__i386) || defined(__i386__) || defined(_M_IX86)
    #error cmake_arch i386
#elif defined(__x86_64) || defined(__x86_64__) || defined(__amd64) || defined(_M_X64)
    #error cmake_arch x86_64
#endif

This strategy is also the recommended one for detecting the target processor architecture, where CMake does not seem to offer a portable intrinsic solution.

Yet another alternative exists. It will only use CMake, doing away entirely with the preprocessor, at the expense of having a different source file for each case, which would then be set as the source file for the executable target arch-dependent using the target_sources CMake command:

add_executable(arch-dependent "")

if(CMAKE_HOST_SYSTEM_PROCESSOR MATCHES "i386")
  message(STATUS "i386 architecture detected")
  target_sources(arch-dependent
    PRIVATE
      arch-dependent-i386.cpp
    )

elseif(CMAKE_HOST_SYSTEM_PROCESSOR MATCHES "i686")
  message(STATUS "i686 architecture detected")
  target_sources(arch-dependent
    PRIVATE
      arch-dependent-i686.cpp
    )
elseif(CMAKE_HOST_SYSTEM_PROCESSOR MATCHES "x86_64")
  message(STATUS "x86_64 architecture detected")
  target_sources(arch-dependent
    PRIVATE
      arch-dependent-x86_64.cpp
    )
else()
  message(STATUS "host processor architecture is unknown")
endif()

This approach will clearly require more work for an existing project, since the source files will need to be separated. Moreover, code duplication between the different source files might certainly become a problem.