Currently we build-depend on the native python (via asciidoc), and on
the host python (via python-dev). As these are not coinstallable it
is impossible to perform a complete cross-build. Until that's resolved,
this will allow cross-building of most of the package with the
combination of the 'cross' and 'nopython' profiles.
(This also sidesteps the issue of perf wanting a multilib compiler.)
By passing both -C and -f options to make, we can separate makefiles
from the output directory without adding prefixes to all targets.
We can also reduce the 'clean' commands to little more than 'rm -rf'.