Your radeon driver was probably miscompiled, causing the X server to die due to an unresolved symbol. This would be visible in the X server stderr output, e.g. in the display manager log file.

Does re-running autogen.sh in the xf86-video-ati Git tree and recompiling the driver help?

(In reply to comment #1)
Making the driver clean, re-running autogen.sh and recompiling doesn't help.

The last line of lightdm x-0.log shows an undefined symbol in the radeon driver:
-----
/usr/bin/X: symbol lookup error: /usr/lib/x86_64-linux-gnu/xorg/modules/drivers/radeon_drv.so: undefined symbol: DRI2ScreenInit
-----

Did the xserver build produce libdri2.so? It should be loaded by default, but there's no trace of it in your log file.

Granted, the driver could probably handle this more gracefully, but I assume you want DRI2 anyway.

(In reply to comment #3)

No libdri2.so wasn't compiled. I upgaded dri2proto to the latest git version
and started rebuilding xserver. Unfortunately it no longer builds because of
the following error:
----
glxdri2.c: In function 'dri2_convert_glx_attribs':
glxdri2.c:431:24: error: '__DRI_API_OPENGL_CORE' undeclared (first use in this function)
glxdri2.c:431:24: note: each undeclared identifier is reported only once for each function it appears in
glxdri2.c:474:20: error: '__DRI_CTX_FLAG_DEBUG' undeclared (first use in this function)
glxdri2.c:474:20: error: '__DRI_CTX_FLAG_FORWARD_COMPATIBLE' undeclared (first use in this function)
glxdri2.c:486:9: warning: statement with no effect [-Wunused-value]
glxdri2.c: In function 'initializeExtensions':
glxdri2.c:902:29: warning: cast discards '__attribute__((const))' qualifier from pointer target type [-Wcast-qual]
glxdri2.c: In function '__glXDRIscreenProbe':
glxdri2.c:949:36: warning: cast discards '__attribute__((const))' qualifier from pointer target type [-Wcast-qual]
glxdri2.c:950:24: warning: cast discards '__attribute__((const))' qualifier from pointer target type [-Wcast-qual]
glxdri2.c: At top level:
glxdri2.c:393:1: warning: 'dri2_convert_glx_attribs' defined but not used [-Wunused-function]
make[1]: *** [glxdri2.lo] Error 1
make[1]: Leaving directory `/home/jos/src/xorg/xorg-server-1.12.99.901/glx'
make: *** [all-recursive] Error 1
-----

Looks like you need GL/internal/dri_interface.h from a newer version of Mesa.

(In reply to comment #5)
> Looks like you need GL/internal/dri_interface.h from a newer version of Mesa.

I'm already using the latest git version of mesa, and of drm, libX11, libxcb, libXi, xf86-video-ati and xf86-input-evdev.
/usr/include/GL/internal/dri_interface.h is the same as the one in mesa.

(In reply to comment #6)
> /usr/include/GL/internal/dri_interface.h is the same as the one in mesa.

So, does it contain those defines? If yes, is there another version of the header somewhere else that the build might be picking up? ...

Created attachment 64157 [details]
dri_interface.h from mesa

This is /usr/include/GL/internal/dri_interface.h provided by mesa (git version)

There was also an old version in /usr/local/include/ which I removed
but this doesn't fix the build issue.

This build issue is not new, I previously filed Bug 51140 - git xserver fails to build.
No such issue with xserver-db9d2b8 and previous ones.

I don't see how the xserver build can pick up the dri_interface.h file you attached and fail with the errors in comment #4...

(In reply to comment #10)
It will stay a mystery!
Since today's last git xserver-d5977e5, xf86: include xf86platformBus.h in xf86AutoConfig.c no more build issues.
This bug can be closed.