Fix up errors and warnings from g++

src/align.h

src/clocks.c

src/common_io.c

src/common_io.h

src/engine.c

src/gravity_cache.h

src/io_properties.h

src/runner_doiact.h

src/runner_doiact_grav.h

src/single_io.c

src/space.c

src/statistics.c