GCC outputs warnings about C++ flags when compiling the C example.

cc1: warning: command-line option ‘-Wnon-virtual-dtor’ is valid for C++/ObjC++ but not for C
cc1: warning: command-line option ‘-Woverloaded-virtual’ is valid for C++/ObjC++ but not for C
cc1: warning: command-line option ‘-Wold-style-cast’ is valid for C++/ObjC++ but not for C

Using CMake generator expression COMPILE_LANGUAGE to make some flags active only when compiling C++ solves it.