Log Message:
Update glib2 to 2.54.1

glib-genmarshal and glib-menums have been ported to python. To keep
glib2 a python-free zone, these have been moved with gdbus-codegen
to py-glib2-tools.

Packages needing one of those tools to build can include
devel/glib2/buildtools.mk

The licence has been changed from LGPLv2+ to LGPLv2.1+.

A full changelog is available at:
https://git.gnome.org/browse/glib/tree/NEWS?h=2.54.1