Allow GIT version to be used in pacman builds

Add a new configure flag, --enable-git-version, that allows the output of
'git describe' to be used in the version string associated with this
package. This could aid in debugging for users that are using a development
version of pacman and we should be able to figure out which cut of code they
are using.

Sample output:
$ pacman --version
Pacman v3.1.4-190-g4cfa-dirty - libalpm v2.3.1

$ makepkg --version
makepkg (pacman) 3.1.4-190-g5861-dirty

Signed-off-by: Dan McGee <dan@archlinux.org>

0 files changed, 0 insertions, 0 deletions