Allow to opt-out of automatic msys2_mingw_dependency installation

Hi @jcupitt ,

as discussed in #331 (see comments), we have the need of more fine-grained control on the libvips library version to be used.

When you install the ruby-vips gem on Windows, it would automatically (and unconditionally) install the latest available libvips dependency to your msys2 ruby devkit environment (see here in ruby-vips.gemspec).

There is the %RUBY_DLL_PATH% environment variable which you can point to an external libvips installation (e.g. obtained from libvips/build-win64-mxe releases), but this only works when there is no libvips installation present in the msys2 ruby devkit environment.

So in order to make use of %RUBY_DLL_PATH% to get control of the libvips version being installed, you need to make sure that there is no parallel libvips installation in the msys2 ruby devkit environment. But here's the catch: every time you re-install or update the ruby-vips gem, you will get an msys2 libvips installation as a side effect, breaking the use of the external libvips version in %RUBY_DLL_PATH%.

One solution to this could be to provide a possibility to opt-out of the installation of "msys2_mingw_dependencies" in the gemspec (e.g. via an env var like SKIP_MSYS2_MINGW_DEPENDENCIES=1 or so).

What do you think? Does that sound feasible / sensible? Do you have other suggestions on how to enable better control over the libvips library version?

Cheers,
Torben

Start with ruby-vips.gemspec around line 26 and review how the msys2_mingw_dependencies installation interacts with RUBY_DLL_PATH on Windows. Define an opt-out that preserves external libvips selection without breaking normal gem installation, then verify the gemspec behavior in the Windows installation path.