[Dcmlib] New feature : choose namespace at CMake time

[Jean-Pierre Roux]

Hi !
Just to keep a track in the M.L. :

In order to allow using both ITK (that includes gdcm 1.2) and the 
current version (1.3 or later) developer may choose his owm namespace, 
at the 'GDCM_NAME_SPACE' option , e.g. gdcm13; (default is 'gdcm')

Jean-Pierre Roux