Fix the CMake for GCC 4.6

 - GCC 4.4, 4.5 and 4.6 uses the flag "-std=c++0x" instead of "-std=c++11"
 - GCC 4.7 and above can use any two of them

NOTE: this is only part of a fix for building against GCC 4.6 ; I will also provide a fix for the source code
NOTE: GCC 4.6 is used under Ubuntu 12.04 current LTS, and so it is the default for Travis Continuous Integration free services

Cheers,
SRombauts