Fixes libarchive v3.1.2 (via Somasis) and Python v3.5.1 build failures.
    - Adds nl(1) to ${CHECK_PREREQ_CMDS}.
    - Adds `Exherbo Linux (x86_64-pc-linux-musl) (x86_64)' to TESTED_ON
      and build.usage.
    - Build libarchive w/ --without-expat --without-xml2.
    - Install Python w/ prefix=.
    - Make libarchive use slibtool even when installing and remove the
      libtool-related patch and ${PKG_LIBARCHIVE_LIBTOOL_MIDIPIX}.