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
PRG_GIT_REVISION := $(shell git describe)
PRG_GIT_DATE := $(shell git log -n 1 --format=\"%cd\")
PRG_GIT_URL := "https://github.com/dstndstn/astrometry.net"

CFLAGS_DEF += -DPRG_GIT_REVISION=\"$(PRG_GIT_REVISION)\"
CFLAGS_DEF += -DPRG_GIT_DATE=\"$(PRG_GIT_DATE)\"
CFLAGS_DEF += -DPRG_GIT_URL=\"$(PRG_GIT_URL)\"
```