in order to advise contributors to locally test their patches.

Also update ctest to include a fix on rerun-if-changed so that human
developers doing trial & error can properly test their latest code.

Signed-off-by: NODA, Kai <nodakai@gmail.com>

This adds a `use_std` Cargo feature which disables `#![no_std]` builds of libc,
but is enabled by default. The library will currently continue to link to the
standard library to maintain backwards compatibility with the 0.2 series and
older Rust compilers for now, but this default can possible be changed in the
future.

Now we're testing rumprun, openbsd, and freebsd