features/standards: use standards macros for BSDs (re: 18d24dd)

The features/standards iffe script claims:
/*
 * On BSD systems, _POSIX_SOURCE and such are used to *limit*
 * functionality to a known API; they don't enable anything. The
 * general intent in BSD is to enable everything by default.
 */
But this is completely false. I don't remember where on earth I
ever got that idea. All sorts of stuff is disabled by default and
needs to be enabled using _POSIX_SOURCE, _XOPEN_SORUCE, and (on
OpenBSD) _BSD_SOURCE or (on NetBSD) _NETBSD_SOURCE.

I discovered this when parallel building became impossible on all
the BSDs as of the referenced commit, because the SA_RESTART macro
is not defined by default on any of them. <sigh>

src/cmd/INIT/mamake.c

@@ -139,8 +139,9 @@ static const char usage[] =
 #elif defined(__QNX__) && !defined(_QNX_SOURCE)
 #define _QNX_SOURCE 1
 
-/* Everything else (minus BSD, as the defaults there are acceptable) */
-#elif !(BSD && !__APPLE__ && !__MACH__ && !NeXTBSD) && !defined(_POSIX_C_SOURCE)
+/* Everything else */
+#else
+#define _XOPEN_SOURCE	9900
 #define _POSIX_C_SOURCE 21000101L
 #endif
 

src/lib/libast/features/standards

@@ -33,10 +33,6 @@ set define
 #
 
 if tst note{ BSD (Free, Net, Open, et al) }end compile{
-		/*
-		 * On BSD systems, _POSIX_SOURCE and such are used to *limit* functionality to a known API;
-		 * they don't enable anything. The general intent in BSD is to enable everything by default.
-		 */
 		#include <limits.h>
 		#include <unistd.h>
 		#include <sys/param.h>
@@ -56,7 +52,10 @@ if tst note{ BSD (Free, Net, Open, et al) }end compile{
 			return 0;
 		}
 	}end {
-		/* No standards or features macro here. On BSD, everything is enabled by default */
+		#define _XOPEN_SOURCE	9900
+		#define _POSIX_C_SOURCE 21000101L
+		#define _BSD_SOURCE	1
+		#define _NETBSD_SOURCE	1
 	}
 elif tst note{ Darwin (macOS, Mac OS X) }end compile{
 		/*