headers/bsd: Rewrite link_elf.h.

Include only the APIs we are (shortly) going to actually support.

The other structures and functions declared in this file were
never supported nor used anywhere in Haiku's tree.

Add missing include in bsd/sys/link_elf.h

* Doesn't build without it.

Change-Id: Ia8671f1545fee6684278e688d0a16a1147e78842
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1866
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Add a features.h to auto enable _DEFAULT_SOURCE

Unless __STRICT_ANSI__ is defined (as it is when running the compiler in
--std=c89 or --std=c99, but not when running it without any specific
args), we can enable these by default and behave like most other
systems. I don't know why no one has done this yet despite suggesting it
multiple times and people prefer to #define _BSD_SOURCE manually
everywhere.

Remove all places in our Jamfiles and sources where it had been defined.
_DEFAULT_SOURCE is now enabled by default for all sources of Haiku, since we
let the compiler use GNU extensions (no strict C standard specified on
command line)

Use _DEFAULT_SOURCE as the define name to match current versions of
glibc. Enable it if _BSD_SOURCE is #defined in compiler flags, for
backward compatibility.

Change-Id: I6db04da5f6db437723cdfba3478f5094a69d7727
Reviewed-on: https://review.haiku-os.org/c/1633
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

BSD: Use elf.h from os/kernel

Platform-dependant defines for Elf_*

Change-Id: Ib86554eb497d20e60ad7c75de4321d5b516ad88c
Reviewed-on: https://review.haiku-os.org/c/1635
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Import link.h and link_elf.h from FreeBSD master

Add _D_BSD_SOURCE guarding
Change-Id: I6eefe207b5e7e645cd3aec3b1981aae59793fa98
Reviewed-on: https://review.haiku-os.org/c/1620
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>