How to Make Compose Work in GTK and Qt Apps

I used Gnome for a long time, but preferred vanilla X input method (xim) over the default GTK behaviour. I just set GTK_IM_MODULE and QT_IM_MODULE environment variables to xim, and didn't have any problem with multiple-layout config, Compose key, custom ~/.XCompose and misc:typo typography extensions.

Recent Gnome 3.6 completely screwed keyboard input by shipping a half-baked and buggy ibus and forcing it upon the users. I just couldn't get the keyboard working as well as before (even with ibus disabled), and had to abandon Gnome completely.

Now I use a simple window manager, and configure my keyboard with setxkbmap. While the keyboard works well again, I couldn't get the Compose key working everywhere. Compose works in plain X11 apps (xterm), but it doesn't work in neither GTK2, nor GTK3, nor Qt apps.

I use current Archlinux versions:

xorg-xinput 1.6.0
xorg-server 1.13.1
xf86-input-keyboard 1.6.2
gtk2 2.24.14
gtk3 3.6.4
qt 4.8.4

and enable Compose key like this:

setxkbmap ... -option 'compose:menu'

and export GTK_IM_MODULE, QT_IM_MODULE variables:

$ echo $GTK_IM_MODULE $QT_IM_MODULE 
xim xim

Compose key is properly recognized by X11:

$ xev | grep -A2 --line-buffered '^KeyRelease' | sed -n '/keycode /s/^.*keycode \([0-9]*\).* (.*, \(.*\)).*$/\1 \2/p'
135 Multi_key

It works properly in xterm, sequences from my ~/.XCompose included. It doesn't work in either GTK or Qt apps. Let say, if I enter Compose ', then apps echo ' immediately, without waiting for the third key in the sequence.

Now I suppose something has changed in either Xorg (because Compose doesn't work in Qt apps either), or in GTK and Qt. *_IM_MODULE variables are not enough now. What else is required to make modern GTK and Qt recognise xim and its Compose sequences?

P.S. There is a similar (and unanswered) question about Compose key not working in GTK. Unlike the asker, I don't have ibus installed.