Turn up warnings to 11

* Turn on some GCC warnings and treat them as errors. For simplicity,
  drop Clang from the Makefile since I'm (probably) using GCC-specific
  flags now.
* Remove stdbool.h from calc.c - it isn't used.
* Use size_t instead of int where appropriate.
* Put a space after sizeof for |sizeof (type)|, to avoid it looking like
  a function call.