Replace deprecated SVN fields by GIT fields collected from git describe ...

Replace deprecated SVN fields with GIT fields (strings) which are provided via CFLAGS macros.

```shell
AN_GIT_REVISION := $(shell git describe)
AN_GIT_DATE := $(shell git log -n 1 --format=\"%cd\")
AN_GIT_URL := "https://github.com/dstndstn/astrometry.net"

CFLAGS_DEF += -DAN_GIT_REVISION=\"$(AN_GIT_REVISION)\"
CFLAGS_DEF += -DAN_GIT_DATE=\"$(AN_GIT_DATE)\"
CFLAGS_DEF += -DAN_GIT_URL=\"$(AN_GIT_URL)\"
```