If proxy-libintl has already been configured before we get to glib, we
will pick that up in dependency('intl'), which then does compiler
checks on it. This was written to assume that the first check will not
find a subproject for libintl, so force it with allow_fallback: false.

Also update the proxy-libintl wrap file and get rid of the explicit
subproject() call.

Reported by Benjamin Gilbert at
!3172