build: allow libintl to be a sibling subproject

Meson 0.59+ has built-in handling of dependency('intl'). Use that if available and cc.find_library() otherwise.

In the case where libintl (specifically proxy-libintl) and gdk-pixbuf are both built as subprojects of the same parent project, we can't check for exported functions at setup time because libintl hasn't been built yet. Detect that case and assume bind_textdomain_codeset() is available.