linux: Specify -std=c++11 when building

[kbeckmann]

GCC 11 updated its default c++ standard from 11 to 17 which caused issues related to std::byte among other things.

I made it build by specifying -std=c++11.

I also renamed CC/CFLAGS to CXX/CXXFLAGS to be more specific that c++ is being compiled.

[trilomix]

Hello kbeckmann,

I didn't merge your fix as there is a conflict with old gcc so I need to introduce gcc dependent compilation...
On the TODO list...

[tcal-x]

Thanks @kbeckmann, I hit the std::byte issue as well and this patch allowed me to build.