There is no /usr/lib/xorg/modules/extensions/glx.so.
The output of ldd /usr/lib/xorg/modules/extensions/libglx.so is:
skel@apsis:~$ sudo ldd /usr/lib/xorg/modules/extensions/libglx.so
linux-vdso.so.1 (0x00007fffbebfc000)
libGL.so.1 => /usr/local/lib/libGL.so.1 (0x00007f2136010000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
(0x00007f2135df3000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f2135bee000)
libaudit.so.1 => /lib/x86_64-linux-gnu/libaudit.so.1
(0x00007f21359c8000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f21356c7000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f213531e000)
libglapi.so.0 => /usr/local/lib/libglapi.so.0 (0x00007f21350ce000)
libXext.so.6 => /usr/lib/x86_64-linux-gnu/libXext.so.6
(0x00007f2134ebc000)
libXdamage.so.1 => /usr/lib/x86_64-linux-gnu/libXdamage.so.1
(0x00007f2134cb8000)
libXfixes.so.3 => /usr/lib/x86_64-linux-gnu/libXfixes.so.3
(0x00007f2134ab2000)
libX11-xcb.so.1 => /usr/lib/x86_64-linux-gnu/libX11-xcb.so.1
(0x00007f21348b0000)
libX11.so.6 => /usr/lib/x86_64-linux-gnu/libX11.so.6
(0x00007f213456c000)
libxcb-glx.so.0 => /usr/lib/x86_64-linux-gnu/libxcb-glx.so.0
(0x00007f2134355000)
libxcb-dri2.so.0 => /usr/lib/x86_64-linux-gnu/libxcb-dri2.so.0
(0x00007f2134150000)
libxcb.so.1 => /usr/lib/x86_64-linux-gnu/libxcb.so.1
(0x00007f2133f30000)
libXxf86vm.so.1 => /usr/lib/x86_64-linux-gnu/libXxf86vm.so.1
(0x00007f2133d2a000)
libdrm.so.2 => /usr/lib/x86_64-linux-gnu/libdrm.so.2
(0x00007f2133b1d000)
/lib64/ld-linux-x86-64.so.2 (0x00007f2136501000)
libXau.so.6 => /usr/lib/x86_64-linux-gnu/libXau.so.6
(0x00007f2133918000)
libXdmcp.so.6 => /usr/lib/x86_64-linux-gnu/libXdmcp.so.6
(0x00007f2133713000)
