Fix for the following issue:
The configure script detects the 8-digit date based version numbers of
an external xrootd installation. However, in the current development
version of xrootd the version scheme has changed to a more traditional
3-number version number (3.0.0). The current root configure script
identifies this number to be older than all the versions using the
old 8-digit version numbers, and sets all the compilation switches wrong. 
The configure script also has problems with xrootd versions installed
from a git head/master checkout, where the version is given using the
git short hash, e.g.: 
   #define XrdVERSION "untagged-e2acb17"


git-svn-id: http://root.cern.ch/svn/root/trunk@37166 27541ba8-7e3a-0410-8455-c3a389f83636