Missing env vars in build script

[FHTMitchell]

Many env vars are missing in the build scripts ($PY_VER, $STDLIB_DIR, $SP_DIR) if no dependencies.host section is defined but python {{ python }} is in dependencies.build. The docs state that if there is no depencies.hosts, then it is assumed to be the same as dependencies.build but here it is not the case. You can see this in the source here.