tools: fix python version checking issue

tools: fix python version checking issue

Even if python version is 2.4.3 which is newer than the required
version 2.3, the configure script still raises a version issue.  I
tested my patch with python 2.6.6 and 2.4.3. It will fix a syntax
error like the following.

 checking for python version >= 2.3 ... Traceback (most recent call last):
   File "<string>", line 1, in ?
 TypeError: 'str' object is not callable
 no
 configure: error: Python 2.4.3 is too old, minimum required version is 2.3

Signed-off-by: Yongjie Ren <yongjie.ren@intel.com>
Acked-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>