The problem is not in the particular code: ni fact, I can't compile any C++ code on this machine! I guess I'll have to reformat the volume and re-install everything... Any better idea is welcome!

I've done some comparisons between my two Macs, one of which couldn't compile anything. It occurred from time to time on the latter that some included header was not found, but launching make again succeeded. Recently, clang++ started to always fail on that machine at the first include it met, while Xcode hadn't that problem. Toggling between uninstall and install of Xcode and the CLI tools didn't help. Apple supplies two distinct SDK sets: one embedded in Xcode, and the other for use by the CLI tools. For some reason, the latter recently got confused as to where to find the headers when Xcode was also installed on the machine. The (temporary?) solution has been for me to uninstall Xcode, keeping only the CLI tools. A matter for Apple guys to think over...