CMake 3.31.3 no longer detects Boost 1.66 due to Config mode preference

[Daisuke Fujimura]

When using the current Cygwin CMake package (3.31.3), CMake fails to
detect an installed Boost 1.66 installation.
This issue appears to be caused by an upstream change introduced
around CMake 3.30, where find_package(Boost) now prefers Config mode
over the legacy Module mode (FindBoost.cmake).

Since Boost 1.66 does not provide a BoostConfig.cmake, CMake 3.31.3
assumes that Boost is not installed, even though the libraries are
actually present.

As far as I understand, this is an expected upstream behavior rather
than a Cygwin packaging bug. However, for users still depending on
older Boost versions, it may cause unexpected detection failures.

Upgrading Boost to version 1.70 or later (which provides
BoostConfig.cmake) is therefore recommended.

I’m aware that the Boost package currently has no active maintainer,
but I wanted to document this behavior in case others encounter the
same issue.