Support for Cygwin toolchain.

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Also tested with the latest version running on Windows XP (gcc-5.4.0)
(http://www.crouchingtigerhiddenfruitbat.org/cygwin/timemachine.html).

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It seems that the open function is declared in <fcntl.h>, not in <unistd.h> (https://www.man7.org/linux/man-pages/man2/open.2.html).

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It seems that for the Cygwin toolchain, just like the Linux toolchain, we should not pass -lsocket to the linker.

[DoctorNoobingstoneIPresume]

Regarding CMake scripts:

I wonder why quotes are needed around ${CMAKE_SYSTEM_NAME} for the MATCHES operator, but not for the STREQUAL operator. (I could see that already quotes have indeed been used with MATCHES. I could also see that, without quotes, MATCHES does not seem to work as expected.)

Anyway, maybe it is useful (good practice, good model) to use quotes (almost) everywhere around variable names. For example, should/might we replace ${CMAKE_SYSTEM_NAME} with "${CMAKE_SYSTEM_NAME}" everywhere ?