CMake Checking Platform

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There are several different ways to check the current system/platform in CMake, and each interacts with cross-compiling in different ways.

Platform Variables

CMake sets certain variables to true depending on the current platform and toolchain in use. These always describe the target platform.

UNIX 
is TRUE on all UNIX-like OS's, including Apple OS X and CygWin
WIN32 
is TRUE on Windows. Prior to 2.8.4 this included CygWin
APPLE 
is TRUE on Apple systems. Note this does not imply the system is Mac OS X, only that __APPLE__ is #defined in C/C++ header files.
MINGW 
is TRUE when using the MinGW compiler in Windows
MSYS 
is TRUE when using the MSYS developer environment in Windows
CYGWIN 
is TRUE on Windows when using the CygWin version of cmake

CMake System

A cleaner and more cross-compiling-compatible way to check the current platform is through the CMAKE_SYSTEM variables.

CMAKE_SYSTEM 
the complete system name, e.g. "Linux-2.4.22", "FreeBSD-5.4-RELEASE" or "Windows 5.1"
CMAKE_SYSTEM_NAME 
The name of the system targeted by the build. The three common values are Windows, Darwin, and Linux, though several others exist, such as Android, FreeBSD, and CrayLinuxEnvironment. Platforms without an operating system, such as embedded devices, are given Generic as a system name.
CMAKE_SYSTEM_VERSION 
Version of the operating system. Generally the kernel version.
CMAKE_SYSTEM_PROCESSOR 
the processor name (e.g. "Intel(R) Pentium(R) M processor 2.00GHz")
CMAKE_HOST_SYSTEM_NAME 
The name of the system hosting the build. Has the same possible values as CMAKE_SYSTEM_NAME.