Fix C++11 support when compiling with GCC. Closes #12.

Add the '-std=c++0x' or '-std=c++11' CXX flag to GCC compiler according to its
version.
Enable C++11 flag only for GCC >= 4.7.
Disable C++11 features when CUDA is enabled
Merge pull request #19 from mwiebe/cmake-gnu-compiler

Tweak C++11 flags in cmakelists.txt