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>

Protect all BSD extensions with _BSD_SOURCE

* Do not define the symbols by default, as they are not in the default
libraries.
* Adjust jamfiles of all code using BSD extensions to define
_BSD_SOURCE.
* This makes Haiku slightly more compliant to standard C/POSIX.

* Added definitions for n_short, n_long, and n_time as expected on FreeBSD when
including in_systm.h


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@42468 a95241bf-73f2-0310-859d-f6bbb57e9c96

Minor improvements for the BSD compatibility library:
* Added {get|set|end}usershell() functions.
* Define MAXLOGNAME, and L_SET, L_INCR, and L_XTND.
* The pidfile stuff in libutil.h is now included, too.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@23827 a95241bf-73f2-0310-859d-f6bbb57e9c96

Protect all BSD extensions with _BSD_SOURCE

* Do not define the symbols by default, as they are not in the default
libraries.
* Adjust jamfiles of all code using BSD extensions to define
_BSD_SOURCE.
* This makes Haiku slightly more compliant to standard C/POSIX.

* Added definitions for n_short, n_long, and n_time as expected on FreeBSD when
including in_systm.h


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@42468 a95241bf-73f2-0310-859d-f6bbb57e9c96

Minor improvements for the BSD compatibility library:
* Added {get|set|end}usershell() functions.
* Define MAXLOGNAME, and L_SET, L_INCR, and L_XTND.
* The pidfile stuff in libutil.h is now included, too.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@23827 a95241bf-73f2-0310-859d-f6bbb57e9c96