maint: update the instructions for building prerequisites

* README-prereq: Update as per the latest required versions
in bootstrap.conf.  Also add a missing cd command.
Reported by Aaron Davies
Fixes http://bugs.gnu.org/15612
dircolors: add putty-256color terminal type

* src/dircolors.hin: Add putty-256color
Reported-by: Thomas D. <whissi@whissi.de>, via
http://bugs.gentoo.org/486786
Fixes http://bugs.gnu.org/15624