Be more careful including <sys/time.h>.

As suggested by Olivier Tristan <o.tristan@uvi.net> and others on
the libsndfile-dev mailing list.
configure.ac : Add --enable-sanitizer option.

Currently this includes:
    -fsanitize=address
    -fsanitize-integer

Even with this option enabled, the flags will only be added if the
compiler supports them.
src/* tests/* : Fix problems found by clang sanitizer.
tests/floating_point_test.tpl : Adjust test targets for -fsanitizer=integer.