* src/bddop.c: Empty macro arguments are undefined ISO C90 and
ISO C++98.  Use '+' when calling APPLY_SHORTCUTS.
* src/fdd.c, src/kernel.c: Avoid constructs invalid in C90.
* src/bddop.c, src/bddx.h, src/kernel.c, src/kernel.h,
examples/cmilner/cmilner.c: Remove C++ comments.