message: Change __gcov_flush() to __gcov_dump()
diff: #include "asserts.hpp"
  #ifdef GCOV
  -extern "C" void __gcov_flush();
  +/// Explicitly dump coverage counters.
  +/// https://gcc.gnu.org/onlinedocs/gcc/Gcov-and-Optimization.html
  +extern "C" void __gcov_dump();
  #endif
  namespace fizzy
  @@ -13,7 +15,7 @@ namespace fizzy
  bool unreachable() noexcept
  {
  #ifdef GCOV
  - __gcov_flush();
  + __gcov_dump();
  #endif
  return false; // LCOV_EXCL_LINE
  }
  