Use git commit in version if built from git

[rodarima]

Attempts to fix #288

I see at least one big problem:

$ git clone ... && cd dillo
$ ./autogen.sh # this will populate the version
$ mkdir build && cd build
$ ../configure && make # Okay, not we build dillo with the correct version
$ cd ..
$ $EDITOR src/...
$ git checkout -b foobar
$ git add ...
$ git commit -m foo
$ cd build
$ ../configure # <--- now we are placing the wrong version in dillo, as it was not updated in configure

So if I forget to run ./autogen.sh again I get a fully working dillo binary with the wrong version.

This should run at configure time, not when running autoconf.