[tlbuild] Work/buildinfo.log now created

[Karl Berry]

The TL ./Build script and TL "make world" should now (r51335) output a
new file buildinfo.log in the build directory, e.g., Work/buildinfo.log.

The file is created by a small new script doc/build-info.sh, which tries
to output compiler versions, dumps the environment, runs uname,
config.status, and a few other commands to try to identify the current
build setup. (I hope the printf command is supported widely enough.)

Some of the information is also written at the beginning of the main
Build output (Work/build.log), and the environment is dumped on its own
to Work/buildenv.log.

The script is run in the new build-info target in the top-level
Makefile, which in turn is invoked as the first thing in the 
"make world" (which is what ./Build executes, by default).

build-info.sh can be run on its own, but then any make-level variable
settings, etc., most likely won't be reported (since they typically
aren't set in the environment).

The hope is that we'll be able to use this to (semi-)automatically
record the build setup for the binaries we distribute, as well as for
reporting/debugging system problems.

I tried the script on a few BSD and GNU/Linux systems. It will not run
on Windows and is not intended (or needed) to.  Suggestions welcome for
Unix portability/usefulness ... --thanks, karl.