Transmission since version 1.61 need GTK+ 2.12 or an upper version.
Transmission used the method "gtk_icon_theme_choose_icon()" available since GTK+ 2.12: http://library.gnome.org/devel/gtk/2.11/GtkIconTheme.html
Steps to Reproduce:
1. emerge transmission
build extract highlighting the problem:
/var/tmp/portage/net-p2p/transmission-1.61/work/transmission-1.61/gtk/icons.c:148: undefined reference to `gtk_icon_theme_choose_icon'
Opened upstream: http://trac.transmissionbt.com/ticket/2453
Fixed upstream in trunk by r9194 for 1.80.
This error occurs when you've got a newer version of glib installed -- one that has gio -- and an older version of gtk.
I've bumped the gtk+ dep in ebuilds to 2.12, once 1.80 is out we can use the version specified in configure.ac again.