Additional installation prefix variables

The `install_prefix` variable has been replaced with a handful of new
variables, which allow you to further customize the locations for each
kind of artefact that a built package installs.  The CMake and autotools
builders have been updated to pass the values of those variables into
the corresponding configuration programs.
Allow distros to override default global variables

On 64-bit RPM distros, for instance, we need to update `lib_dir_name` to
`lib64`, so that we install built shared libraries into the right
location.
Merge branch 'feature/lib64' into develop

* feature/lib64:
  Allow distros to override default global variables
  Additional installation prefix variables
RPM spec requires spaces around `>=`

Silly, but true.
Don't build package before uninstalling

That's just silly.
Use yum's `--cacheonly` option

We don't need yum to connect to the network to update all of its repos
every time we want to grab information about a package.  Especially when
we're running inside of a Vagrant VM that has some weird overhead for
each network connection!