Fixed slackware backend.
Got rid of stupid `spmk_local`. Cleaned pkgmode.
Fixed prebuild and postbuild hooks. Simplified package definition.

Autogenerate rules like
    sample-pkg:QV: sample-pkg-1.7.8-1

So build.mk can no longer contain superfluous things such as

    sample_pkg_ver = 1.7.8-1
    sample-pkg:VQ: sample-pkg-$sample_pkg_ver