_Noreturn kw must appear before the function name as per C11 grammar and
clang7 is the first one to enforce that.

Ecb_noreturn macro is defined using _Noreturn keyword if C11 support is
detected; otherwise __noreturn__ gcc attribute is used. The later must
appear after the function parameters list.  These two methods to
annotate a function are inherently incompatible in terms of the token
location relative to the function declaration.

Use gcc attribute exclusively since it is always available.

Explicitly close net.box connection on error.

Partially reverts squashed 316d4e3a

This regression introducing during replacing port_ffi with port_buf
by 2fcce449

Linux-specific option obsolete since kernel 2.2

Wait() no longer called hence waiters not initialized. Does it make
sence to have a test for :waiters (undocumented internals)?

- Waiting on a brand new socket (neither listen nor connect called)
  completes immediately with HUP status on Linux. However on FreeBSD/osx
  it never completes.

- Attempting to connect to a port bound on a loopback interface which is
  not yet listening yield ECONNREFUSED in blocking mode on Linux. On
  FreeBSD/osx blocking connect never completes ("never" means it doesn't
  complete in a reasonable time).

- Calling shutdown() on a listening socket works in Linux but in
  FreeBSD/osx it doesn't.

Note: it was incomplete, made thing unnecessary complex and it did not
align with the CMake way (c) for crosscompilation.

Pass sysroot on OSX.
Make the code in luajit.cmake a bit more sane and straightforward.

Note: v1 of this patch was reverted due to performance regression.