configure.in: change check for 'VERSION' file:

- first check if this file exist and use the content from there
- if it not exist check with 'git log'
- if this fails set to 'Unknown'

This patch also shows the GIT_REV in the configure summary message. To display This message on top of the messages we move the whole block a bit above.

This is a enhancenment to https://github.com/xbmc/xbmc/pull/1934