make sanity RFE

[Jonathan Gibbons]

For the first time, I've been trying a full complete build of OpenJDK
(full open+closed forest.)

I'm building on Ubuntu, and the build has now failed a second time
because of a missing command -- or more specifically, a command which
is not installed by default.  The first command was msgfmt, the latest
was bb. I do not know if there are more to come.

It would be nice if make sanity had a mechanism for a quick existence
check for commands that are likely to be required during the build, so
that missing commands can be detected and reported early, and not
one at a time at the end of a long build.

-- Jon