features.h: Define _DEFAULT_SOURCE when _GNU_SOURCE is defined.

As suggested by PulkoMandy. This was done before my commits yesterday,
but those just reverted patches that had only been in since May,
so it's not clear how much this is actually needed. Nonetheless
it seems like the more correct thing to do.

features.h: Clean up ISOC11 definitions.

* Do not define it merely when _GNU_SOURCE is defined, that is not needed.
* Scale back __cplusplus check to 201103L, just like the BSDs do.

features.h: enable ISOC11 definitions for _GNU_SOURCE and C++17

this is needed for GCC to build after the aligned_alloc introduction

Change-Id: Ieae32a050cc000561107c4a07cf10c912a196152
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3896
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

features.h: undef before define

Change-Id: Iba98e3a8352801345acefe963917471992f70281
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3606
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

C11: add aligned_alloc()

Change-Id: If648c0e27ed946874d393e8e33a4548b70c8ecdb
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3377
Reviewed-by: Adrien Destugues <pulkomandy@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>