add _DEFAULT_SOURCE feature profile as an alias for _BSD_SOURCE

as a result of commit ab8f6a6e42, this
definition is now equivalent to the actual "default profile" which
appears immediately below in features.h, and which defines both
_BSD_SOURCE and _XOPEN_SOURCE.

the intent of providing a _DEFAULT_SOURCE, which glibc also now
provides, is to give applications a way to "get back" the default
feature profile when it was lost either by compiler flags that inhibit
it (such as -std=c99) or by library-provided predefined macros (such
as -D_POSIX_C_SOURCE=200809L) which may inhibit exposure of features
that were otherwise visible by default and which the application may
need. without _DEFAULT_SOURCE, the application had encode knowledge of
a particular libc's defaults, and such knowledge was fragile and
subject to bitrot.

eventually the names _GNU_SOURCE and _BSD_SOURCE should be phased out
in favor of the more-descriptive and more-accurate _ALL_SOURCE and
_DEFAULT_SOURCE, leaving the old names as aliases but using the new
ones internally. however this is a more invasive change that would
require extensive regression testing, so it is deferred.

include/features.h

@@ -5,6 +5,10 @@
 #define _GNU_SOURCE 1
 #endif
 
+#if defined(_DEFAULT_SOURCE) && !defined(_BSD_SOURCE)
+#define _BSD_SOURCE 1
+#endif
+
 #if !defined(_POSIX_SOURCE) && !defined(_POSIX_C_SOURCE) \
  && !defined(_XOPEN_SOURCE) && !defined(_GNU_SOURCE) \
  && !defined(_BSD_SOURCE) && !defined(__STRICT_ANSI__)