[RSB] Build GNU sed if not available on host

[chrisj at rtems.org]

Hi,

These patches add support to build GNU sed if it is not avaliable on
a host. GCC requires GNU sed or weird things happen. Ticket #2300
details a MIPS build failure that has existed in GCC for years
where the multilib processing depends on GNU sed.

The simplest solution is to check and if sed is not GNU build it.
This happens on MacOS and FreeBSD.

The patches also include a fix for the --no-install option.

Chris