Preinstalled packages

On Mac OS X, some packages are preinstalled system packages, which means
that Homebrew (what we're using as the native package manager) doesn't
know about them.  That means they end up looking like they're
uninstalled and not available.  You can now set a
`preinstalled.homebrew.[package]` variable to tell Buzzy about these
preinstalled packages.  The value of the variable should be the version
number that is preinstalled.
Fix unused function warning
Check minimum version for native packages

If a native package is available for some Buzzy dependency, we must make
sure that the native version that's available satisfies any version
constraint in the dependency.
yum -C instead of --cacheonly

The `--cacheonly` long option isn't available in RHEL5's yum version.
The equivalent `-C` is available, however, in the older and newer
versions.
Don't require check test harness

We use check [1] as one of our testing harnesses.  RHEL5 doesn't provide
a new enough version of check for our purposes, though.  We can build
and install a newer version using Buzzy, but not until Buzzy is built:
a dastardly Catch-22.  So now we make all of the buzzy tests optional.
This lets us bootstrap and install a version of Buzzy, which we can then
use to build and install a newer version of check, which we can then use
to go back and run the Buzzy tests.

[1] http://check.sourceforge.net/