Add version requirement >= 2.60 for the GLib family of libraries

    - due to usage of Reference-counted-data in lib/wptoml/array.cpp
      (requires >= 2.58, see https://developer.gnome.org/glib/stable/glib-Reference-counted-data.html)
    - and usage of G_GNUC_FALLTHROUGH in lib/wp/object-interest.c
      (requires 2.60, see https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html#G-GNUC-FALLTHROUGH:CAPS

)

Signed-off-by: Martin Koch, ESE GmbH <martin.koch@ese.de>