if(CMAKE_SYSTEM_NAME MATCHES "Linux")
message("current platform: Linux ")
elseif(CMAKE_SYSTEM_NAME MATCHES "Windows")
message("current platform: Windows")
elseif(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
message("current platform: FreeBSD")
else()
message("other platform: ${CMAKE_SYSTEM_NAME}")
endif(CMAKE_SYSTEM_NAME MATCHES "Linux")或
if(WIN32)
message("Now is windows")
elseif(APPLE)
message("Now is Apple systens.")
elseif(UNIX)
message("Now is UNIX-like OS's. Including APPLE os x and CygWin")
endif()