Add the -std=c++0x flag to the CXX compile options such that it finds std::shared_ptr etc

Why was the line `ADD_DEFINITIONS(-std=c++0x)` commented out before? 

At least GCC 4.8 needs it, and it should not hurt for prior GCC versions (GCC 4.7 builds fine with the flag set). Those that would not know this flag are too old to compile vigra anyway.

Replaced it with `SET(CMAKE_CXX_FLAGS -std=c++0x)` to only add it to CXX and not CC to prevent warnings from the C compiler.

Tested with GCC 4.8 on OSX 10.9 Mavericks, and GCC 4.7 on Debian.
I'd prefer to activate `-std=c++0x` only wgere it is actually needed. Moreover, you should check if the user has already provided a conflicting flag (e.g. `-std=c++11`) which shouldn't be overridden.