Comment 4 for bug 1368168

It seems that this is caused by previously installed fglrx failed to remove x86_64-linux-gnu_gl_conf when it get removed. After removing fglrx, /usr/lib/pxpress/ld.so.conf is still shown in update-alternatives:

$ update-alternatives --display x86_64-linux-gnu_gl_conf
x86_64-linux-gnu_gl_conf - auto mode
  link currently points to /usr/lib/pxpress/ld.so.conf
/usr/lib/pxpress/ld.so.conf - priority 901
  slave x86_64-linux-gnu_10fglrx: /usr/lib/fglrx/10fglrx
  slave x86_64-linux-gnu_amdcccle: /usr/lib/fglrx/bin/amdcccle
  slave x86_64-linux-gnu_amdcccle_desktop: /usr/share/fglrx/amdcccle.desktop
  slave x86_64-linux-gnu_amdccclesu_desktop: /usr/share/fglrx/amdccclesu.desktop
  slave x86_64-linux-gnu_amdconfig: /usr/lib/fglrx/bin/amdconfig
  slave x86_64-linux-gnu_amdnotifyui: /usr/lib/fglrx/bin/amdnotifyui
  slave x86_64-linux-gnu_amdocl32.icd: /usr/lib/fglrx/etc/OpenCL/vendors/amdocl32.icd
  slave x86_64-linux-gnu_amdocl64.icd: /usr/lib/fglrx/etc/OpenCL/vendors/amdocl64.icd
  slave x86_64-linux-gnu_amdupdaterandrconfig: /usr/lib/fglrx/bin/amdupdaterandrconfig
  slave x86_64-linux-gnu_amdxdg_su: /usr/lib/fglrx/bin/amdxdg-su
  slave x86_64-linux-gnu_ati_conf: /usr/lib/fglrx/etc/ati
  slave x86_64-linux-gnu_aticonfig: /usr/lib/fglrx/bin/aticonfig
  slave x86_64-linux-gnu_atieventsd: /usr/lib/fglrx/bin/atieventsd
  slave x86_64-linux-gnu_atiodcli: /usr/lib/fglrx/bin/atiodcli
  slave x86_64-linux-gnu_atiode: /usr/lib/fglrx/bin/atiode
  slave x86_64-linux-gnu_clinfo: /usr/lib/fglrx/bin/clinfo
  slave x86_64-linux-gnu_fgl_glxgears: /usr/lib/fglrx/bin/fgl_glxgears
  slave x86_64-linux-gnu_fglrx_dri: /usr/lib/fglrx/dri/fglrx_dri.so
  slave x86_64-linux-gnu_fglrx_drv: /usr/lib/fglrx/xorg/modules/drivers/fglrx_drv.so
  slave x86_64-linux-gnu_fglrx_modconf: /lib/fglrx/modprobe.conf
  slave x86_64-linux-gnu_fglrxinfo: /usr/lib/fglrx/bin/fglrxinfo
  slave x86_64-linux-gnu_grub_fb_blacklist: /usr/share/fglrx/fglrx-updates.grub-gfxpayload
  slave x86_64-linux-gnu_libAMDXvBA_cap: /usr/lib/fglrx/libAMDXvBA.cap
  slave x86_64-linux-gnu_libOpenCL.so: /usr/lib/fglrx/libOpenCL.so
  slave x86_64-linux-gnu_libaticalcl.so: /usr/lib/fglrx/libaticalcl.so
  slave x86_64-linux-gnu_libaticalcl.so_lib32: /usr/lib32/fglrx/libaticalcl.so
  slave x86_64-linux-gnu_libaticalrt.so: /usr/lib/fglrx/libaticalrt.so
  slave x86_64-linux-gnu_libaticalrt.so_lib32: /usr/lib32/fglrx/libaticalrt.so
  slave x86_64-linux-gnu_xorg_extra_modules: /usr/lib/pxpress/xorg
/usr/lib/x86_64-linux-gnu/mesa/ld.so.conf - priority 500
  slave x86_64-linux-gnu_xorg_extra_modules: /usr/lib/x86_64-linux-gnu/xorg/x11-extra-modules
Current 'best' version is '/usr/lib/pxpress/ld.so.conf'.

And this triggers the error in #3 when postinst of fglrx-core is executed.

To manually fix this issue, please do
$ sudo update-alternatives --remove x86_64-linux-gnu_gl_conf /usr/lib/pxpress/ld.so.conf