Workaround Gentoo's silly Automake wrapper in bootstrap.

It's pretty annoying when running bootstrap doesn't actually rebuild
using newly-installed tool versions.  This happens because Gentoo's
automake wrapper script detects that stuff has been previously built
with a particular version of Automake, and runs that instead of the
latest.

Since we're going to recreate them anyway, delete the generated files so
that Gentoo's script picks the latest.