Fix if() checks of CMAKE_SYSTEM_NAME on Cygwin

The CMAKE_SYSTEM_NAME is "CYGWIN", but we also define a variable
named "CYGWIN" to "1".  Avoid allowing if() to expand the "CYGWIN"
string as a variable.
Merge topic 'if-sanity' into next

704e1fa7 Fix if() checks of CMAKE_SYSTEM_NAME on Cygwin