Because we are compiling .c files containing C++ code, clang++ complains
with:
clang: error: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated
If renaming all the source files to .cpp is out of the question, an
alternative is to pass "-x c++" to convince the compiler that we are
really compiling C++. It works fine with GCC too.