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.

headers/compatibility/bsd/paths.h

@@ -37,6 +37,10 @@
 #ifndef _PATHS_H_
 #define	_PATHS_H_
 
+
+#ifdef _BSD_SOURCE
+
+
 #include <sys/cdefs.h>
 
 /* Default search path. */
@@ -136,4 +140,8 @@ __END_DECLS
 #define	_PATH_WALL	"/rescue/wall"
 #endif /* RESCUE */
 
+
+#endif
+
+
 #endif /* !_PATHS_H_ */