Fix Cygwin cairo support and add MinGW cairo support

[jmid]

This PR

• fixes the existing Cygwin cairo support and
• adds MinGW cairo support, modeled after the template PR Update conf-libssl for the mingw-w64 ports #26072

Previously, the conf-cairo-file would install a Cygwin package https://cygwin.com/packages/summary/libcairo-devel.html (and thus depending on cygwin1.dll) rather than a MinGW package, allowing us to build standalone, native Windows executables.

Recall: Windows is supported in multiple forms (also summarized in our wiki-page: https://github.com/ocaml/opam-repository/wiki/Depexts-os-distribution---os-family-values):

• MinGW - which allows to produce native Windows executables and can be installed either through
  • Cygwin (os = "win32" & os-distribution = "cygwin") or
  • MSys2 (os = "win32" & os-distribution = "msys2")
• Cygwin (os = "cygwin" & os-distribution = "cygwin") which produces executables that require Cygwin
• cygwinports (os = "win32" & os-distribution = "cygwinports") is the old, now discontinued approach based on a dedicated Windows opam-repo
• ...

Previously, CI would be green since the Cygwin package exists and installs cleanly - when matching the Cygwin-hosted MinGW environment we currently test in the GitHub actions CI workflow. The CI didn't catch the subtle, unwanted cygwin1.dll dependency introduced though... 🤷

The MinGW packages installed are

• https://cygwin.com/packages/summary/mingw64-x86_64-cairo.html
• https://cygwin.com/packages/summary/mingw64-i686-cairo.html
• https://packages.msys2.org/base/mingw-w64-cairo

and the Cygwin package in question is

• https://cygwin.com/packages/summary/libcairo-devel.html

We don't have CI support for testing pure Cygwin (non-MinGW) ATM, but as in the previous PRs I've tested the two others with a self PR: jmid#13