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 <[email protected]>

headers/compatibility/bsd/paths.h

@@ -37,8 +37,9 @@
 #ifndef _PATHS_H_
 #define	_PATHS_H_
 
+#include <features.h>
 
-#if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
+#if defined(_DEFAULT_SOURCE) || defined(_GNU_SOURCE)
 
 
 #include <sys/cdefs.h>
@@ -141,7 +142,7 @@ __END_DECLS
 #endif /* RESCUE */
 
 
-#endif /* _BSD_SOURCE || _GNU_SOURCE */
+#endif /* _DEFAULT_SOURCE || _GNU_SOURCE */
 
 
 #endif /* !_PATHS_H_ */