remove hard-coding /lib/systemd , for build archlinux package

[hitsmaxft]

archlinux keep /lib/ own by filesystem , pkg shuould not install under /lib

this pr remove hardcoding /lib/systemd in .service file and meson build scripts , and using PKGPREFIX='/lib/systemd/' as default ,

Directory Symlink Handling: Example time! Arch Linux has a /lib -> /usr/lib symlink. Previously, if pacman was installing a package and it found files in /lib, it would follow the symlink and install it in /usr/lib. However the filelist for that package still recorded the file in /lib. This caused heaps of difficulty in conflict resolving – primarily the need to resolve every path of all package files to look for conflicts. That was a stupid idea! So now if pacman sees a /lib directory in a package, it will detect a conflict with the symlink on the filesystem. If you were using this feature to install files elsewhere, you probably need to look into what a bind mount is! Note that this change requires us to correct the local package file list for any package installed using this mis-feature, so we bumped the database version. Upgrade using pacman-db-upgrade. Thanks to Andrew!

ref http://allanmcrae.com/2014/12/pacman-4-2-released/