Link the main library with -Wl,-z,nodelete to prevent unloading

[kleisauke]

See: libvips/php-vips-ext#44

[lovell]

The musl libc already (correctly) considers dlclose a no-op, so this change will bring glibc-based flavours of Linux up to par. This should also remove a problem in sharp for people using it with Node.js "worker threads", which uses reference counting before calling dlclose but can end up doing so too early on a child thread rather than the parent thread.