openocd: avoid documentation rebuild to fix build issues

On PowerPC64(le), we patch all configure scripts. Due to this, the
version.texi in OpenOCD files gets regenerated, and then since it has
a newer date than openocd.info, openocd build system rebuilds the
documentation. Unfortunately, this documentation rebuild fails on old
machines.

We work around this by faking the date of the generated version.texi
file, to make the build system believe the documentation doesn't need
to be regenerated.

Fixes:

  http://autobuild.buildroot.net/results/3cbe65a46e75b8e67846d593884c96df97dec7a4



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>