Since the most recent upgrade, Steam fails to start when STEAM_RUNTIME=0 is in the environment: Each time I start, I get an xterm with the following error before logging in:

Couldn't set up the Steam Runtime. Are you running low on disk space?
Continuing...
Press enter to continue: 

The same message appears again after the full steam windows appeared.
You can find the console output at https://gist.github.com/diafero/4723978.

Leaving STEAM_RUNTIME at its default works fine, i.e. the runtime is actually fully functional. Still, I'd prefer Steam to use my system libraries instead of replacing everything and the kitchen sink.

System information:

Processor Information:
    Vendor:  GenuineIntel
    Speed: 2501 Mhz
    4 logical processors
    2 physical processors
    HyperThreading:  Supported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Supported
    SSE4a:  Unsupported
    SSE41:  Supported
    SSE42:  Supported

Network Information:
    Network Speed:  

Operating System Version:
    Debian GNU/Linux 7.0 (wheezy) (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.7.6
    X Server vendor:  The X.Org Foundation
    X Server release:  11204000

Video Card:
    Driver:  Tungsten Graphics, Inc Mesa DRI Intel(R) Sandybridge Mobile x86/MMX/SSE2

    Driver Version:  3.0 Mesa 8.0.5
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 59 Hz
    VendorID:  0x10de
    DeviceID:  0xde9
    Number of Monitors:  1
    Number of Logical Video Cards:  2
    Primary Display Resolution:  1920 x 1200
    Desktop Resolution: 1920 x 1200
    Primary Display Size: 20.39" x 12.76"  (24.02" diag)
                                            51.8cm x 32.4cm  (61.0cm diag)
    Primary VRAM Not Detected

Sound card:
    Audio device: Intel CougarPoint HDMI

Memory:
    RAM:  3875 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_GB.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  187740 Mb
    Largest Free Hard Disk Block:  87917 Mb

Installed software:

Recent Failure Reports: