add cygwin to supported  cmake targets

Cmake can be used to build on cygwin now.

Note that we can't use
${CMAKE_SYSTEM_NAME} here (with braces),
since that leads to CYGWIN, which is interpreted by cmake
as ${CYGWIN}, which is 1 (true), rather than "CYGWIN".
remove braces in "MATCHES" cmake expressions
Merge pull request #2 from JonnyJD/cmake_cygwin

add cygwin to supported  cmake targets