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

[郑文辉(Techlive Zheng)]

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.