build: make makeinfo optional

This means the user does not have to install texinfo to build OpenOCD.

Change-Id: Id9f42da798d3c2b79e95214c9e2559cf32802251
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2325
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>