Fix all compilation warnings

cc1: warning: command line option ‘-std=c++11’ is valid for C++/ObjC++ but not for C

This happens multiple times during the build process.
However, I hope that the cleanup crew project will solve this in the process.
If not, it should be addressed eventually. I'm not sure if the line needs to be removed or the compiler needs to stop treating those pieces as C (I believe the latter).