[build2] Build tools for build-time dependencies

[Boris Kolpackov]

Now that we have support for build-time dependencies, we've packaged a
few base tools to go with it:

xxd         Hexdump utility for embedding binary data into C/C++ programs.
byacc       Berkeley yacc, reasonably compatible with GNU bison.
reflex      A variant of flex, reasonably compatible with "new" flex.
openbsd-m4  POSIX m4 macro processor that supports many GNU m4 extensions.

For details, see:

https://build2.org/blog/build-tools-build-time.xhtml