Log Message:
glibmm2.68: updated to 2.80.0

2.80.0 (stable):

Glib:
* Add wide_from_utf8() and wide_to_utf8()
* DateTime: Add create_from_local_usec(), create_from_utc_usec()
  and to_unix_usec().

Gio:
* Application: Add get/set/property_version()
* ApplicationCommandLine: Add done()
* DBus::Message: Add get_arg0_path()
* Socket: Add receive_bytes() and receive_bytes_from()
* content_type_guess(): Remove most of an unnecessary overload

Build:
* Require glib-2.0 >= 2.79.2