Fix libvips submodule building

[lopcode]

What

For some reason, building the libvips submodule is broken. It gets as far as ~522 operations and then freezes with output that looks like this:

[552/554] Linking target cplusplus/libvips-cpp.42.dylib
ld: warning: ignoring duplicate libraries: '-lz'

Desired outcomes

Running ./setup_development.sh should:

• Make sure that the submodule is set to the desired libvips version, with no changes
• Compile libvips to the release folder

Anything using libvips, in vips-ffm, should point at this submodule instead of the hardcoded Homebrew path that I've used as a workaround.