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>

build: Deprecate SetSubDirSupportedPlatformsBeOSCompatible.

By default, all targets support the "haiku" platform, and we no longer
support building for BeOS, Dan0, Zeta, or other BeOS-compatible targets,
so this is no longer needed.

Also remove all references to the non-Haiku compatible platforms, and
change all BEOS_COMPATIBLE checks to HAIKU_COMPATIBLE. Removal of
all SetSubDirSupportedPlatformsBeOSCompatible invocations
will be in the next commit.

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.

* Ported FreeBSD's ftpd to Haiku.
* Added it to the image, and configured it to be used.
* Currently, it cannot transfer files over 64 KB for some reason
("Message too long").


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@23843 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.

* Ported FreeBSD's ftpd to Haiku.
* Added it to the image, and configured it to be used.
* Currently, it cannot transfer files over 64 KB for some reason
("Message too long").


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@23843 a95241bf-73f2-0310-859d-f6bbb57e9c96