Revise logic for unified (semistatic) libvips binaries

[kleisauke]

Context:

pyvips could maybe use something similar, I think at the moment it'll try to load glib as a test for semistatic, but that could break on windows if the user has glib from another project on their PATH. Seeing if we can get g_free or whatever from libvips ought to be safer.
Originally posted by @jcupitt in libvips/ruby-vips#390 (comment)