Fix build on illumos (re: 7fb814e1, 580ff616)

The standards macros consistency fix for iffe exposed breakage on
illumos: the standards flags aren't set properly. Back in 580ff616,
I set _XPG6 from features/common, which is the wrong place; the
correct place is features/standards -- especially now that iffe
uses its results.

In addition, to get header declarations that aren't somehow in
conflict with themselves on illumos, don't result in "implicit
function declaration" warnings, and expose all the functionality,
we need to define *all* the _XPG[4-7] macros *and* __EXTENSIONS__
*and* _XOPEN_SOURCE. Welp. Thankfully, that's just fine with
Solaris, too.

Thanks to @JohnoKing for the heads-up.

src/lib/libast/features/common

@@ -36,12 +36,6 @@ cat{
 	#	define	__OPTIMIZE_SIZE__	1
 	#endif
 
-	/* enable POSIX standard on Solaris */
-	#ifdef __sun
-	#	undef	_XPG6
-	#	define	_XPG6		1
-	#endif
-
 	/* __STD_C indicates that the language is ANSI C or C++ */
 	#if !defined(__STD_C) && __STDC__
 	#	define	__STD_C		1

src/lib/libast/features/standards

@@ -19,6 +19,18 @@ if tst note{ FreeBSD or DragonFly BSD }end compile{
 		#endif
 	}end {
 	}
+elif tst note{ SunOS (Solaris, illumos) }end compile{
+		#if !__sun
+		#error dark
+		#endif
+	}end {
+		#define _XPG7		1
+		#define	_XPG6		1
+		#define	_XPG5		1
+		#define _XPG4		1
+		#define	__EXTENSIONS__	1
+		#define	_XOPEN_SOURCE	9900
+	}
 elif tst note{ _ALL_SOURCE & _POSIX_SOURCE & _POSIX_C_SOURCE & _XOPEN_SOURCE & _GNU_SOURCE & _DARWIN_C_SOURCE & __EXTENSIONS__ works }end compile{
 		#define _ALL_SOURCE	1
 		#define _POSIX_SOURCE	1