[pacman-dev] More flexibility on *-vcs building

Now, we can specify 'fragment' parameter to the source url of a VCS package to build a specific version, I wonder if it is possible to make this parameter also accept environment variables, and let `makepkg` handles a depends array elememnt in PKGBUILD like `*-vcs=SPECVIED-vesion` automatically. This way, we can build the a static version of a package but use the source fetched from git.