Add a new feature to enable this, since `const extern fn`
support is unstable

Related to #657 .

See if it works with Cirrus CI!

Reported-by: Alex Zepeda

Closes #651 .

Closes #651 .

added Documentation, Latest Version, License badges similar to rand crate. https://crates.io/crates/rand

Add sparc64-unknown-linux-gnu and x86_64-sun-solaris.
Remove aarch64-unknown-linux-musl.

Signed-off-by: Tom Kirchner <tjk@amazon.com>
Signed-off-by: Ben Cressey <bcressey@amazon.com>

We now create an additional binary `linux_fcntl` for testing this
since there are header conflicts when including all necessary headers.
This binary is run on all platforms even though it's empty on all non-
Android/non-Linux platforms.

Testing has been switched from a custom binary to using a runner-less
test (or pair of tests). This means that for local development a simple
`cd libc-test && cargo test` will run all the tests. CI has also been
updated here to reflect that.

Anyone can do it!

* Rework of original patches from Niels Sascha Reedijk
  that include style and build fixes for libc master

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>