void __gcov_flush();
Since the code is compiled as C++, this declares the existence of a C++ function of that name. C++ functions are subject to name mangling, so the (C++) symbol is not found in the (C) link library, and the linker (rightfully) complains about it.
If you declare the function, declare it as a function with C linkage:
extern "C" void __gcov_flush();
This should do the trick.
Note the commend by Paweł Bylica -- __gcov_flush()
has been removed in GCC 11, you should use __gcov_dump()
.