Fix assorted warnings that are generated in optimized builds.
Most of it is silencing variables that are set but unused.

This patch also introduces the MY_ASSERT_UNREACHABLE macro
which helps the compiler to deduce that a certain piece of
code is unreachable.

include/my_compiler.h:
  Use GCC's __builtin_unreachable if available. It allows
  GCC to deduce the unreachability of certain code paths,
  thus avoiding warnings that, for example, accused that a
  variable could be used without being initialized (due to
  unreachable code paths).