Fix release and standards build flags (re: 35672208, aa4669ad)

bin/package, src/cmd/INIT/package.sh:
- CCFLAGS overwrites the autodetected optimisation flags (e.g. -Os)
  if set. Unfortunately, that also happened when we added something
  to CCFLAGS for a release build or to add an extra flag needed by
  Solaris. The fix is to use a new flags variable (KSH_RELFLAGS)
  instead. This needs to be done in a different place as it needs
  to be added to the mamake command as an assignment argument.
- Remove the Solaris CCFLAGS hack; see features/common below.

src/*/*/Mamfile:
- Add ${KSH_RELFLAGS} to all the compiler commands.

src/lib/libast/features/common:
- Enable POSIX standard on Solaris (i.e.: if __sun is defined) by
  defining _XPG6 directly in the feature test that generates
  ast_std.h, which is indirectly included by everything. This
  removes the need to pass -D_XPG6 via CCFLAGS. (Doing so
  automatically with gcc was not otherwise possible.)

src/cmd/INIT/cc.sol11.*:
- No longer pass -D_XPG6, as per above.

src/lib/libast/features/common

@@ -30,6 +30,12 @@ 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