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I think this comes from here:

I think this comes from here: https://github.com/LibRaw/LibRaw/blob/b31fa58eea272c4ba67ccdbd527f329a5a...

_WIN32 is defined even on Cygwin, but I guess one shouldn't set LIBRAW_WIN32_CALLS on Cygwin; as a consequence, unistd.h is not included from internal/defines.h: https://github.com/LibRaw/LibRaw/blob/master/internal/defines.h#L50-L65

Any reason you're building for Cygwin, and not native MinGW instead?