CMake: only pass 'YY_NO_UNISTD_H' to Flex when unistd.h is not found

CMake: only pass 'YY_NO_UNISTD_H' to Flex when unistd.h is not found

This macro tells Flex that #including unistd.h should be avoided. There is no
need for this on almost any platform except Windows, hence why `YY_NO_UNISTD_H`
is only mentioned elsewhere in the MS Build build system, not in the Autotools
build system. In the CMake build system which is meant to be used across Windows
and non-Windows platforms, we can do something more nuanced and depend on the
existence check of unistd.h itself.