[racket] package builds, updated daily

The site

  http://pkg-build.racket-lang.org/

reports the result of a daily build of all packages that are listed at
"pkgs.racket-lang.org". The builds use the current release of Racket
(version 6.1), and the site includes documentation for packages that
build successfully.

We expect to get these results --- especially links to the built
documentation --- added to "pkgs.racket-lang.org" in the near future.


One caveat: A package fails to install if it depends on a PLaneT
package, even if it references the compatibility variant from
"planet-compats.racket-lang.org". That limitation could be removed with
more work, but I haven't done that work because I think it's better to
get all packages that we care about onto "pkgs.racket-lang.org".


The packages are not built from scratch each day. Instead, each package
that has changed is rebuilt, along with any package that declares a
dependency on a changed package. The build service relies on
virtual-machine instances to sandbox package builds. The implementation
of the build service is `meta/pkg-build/main` in the Racket repo's
"plt-services" package, and I'm happy to provide more details if anyone
is interested.