build: Use git version for tarball and AC_INIT().

build: Use git version for tarball and AC_INIT().

It's kind of silly to use "git" as the version string at all times
except for releases. This commit changes the name of the generated
tarball to use the output of git describe. Hence the "tarball"
target creates a tarball named according to the current git HEAD. For
compatibility we provide a symlink with the old name.