Use GLib's i18n support instead of copying and pasting that
logic into its own header. This deprecates the vips/intl.h
header in favour of glib/gi18n.h.
We had exception specs on the C++ interface, but C++11 does not support
them and some compilers have never honoured them. Remove all specs.
Thanks Lovell.
See https://github.com/jcupitt/libvips/issues/362